Add support for out of tree LayerImpl implementation

[dnicoara]

Problem

Embedders currently need to add local patches in order to provide an out of tree implementation of SystemLayerImpl.

Change overview

This change adds a GN flag allowing embedders to define CHIP_SYSTEM_LAYER_IMPL_CONFIG_FILE to point to whatever file they want.

Testing

gn gen and build without defining chip_system_layer_impl_config_file to verify that default configuration does not break.
gn gen and build with chip_system_layer_impl_config_file to verify that the file is correctly customized.

[github-actions]

PR #14452: Size comparison from 33c4854 to 90fca37

Increases (7 builds for cyw30739, efr32, esp32, p6, qpg, telink)

platform	target	config	section	33c4854	90fca37	change	% change
cyw30739	light	cyw930739m2evb_01	(read/write)	580794	580798	4	0.0
			.app_xip_area	485592	485596	4	0.0
	lock-app	CYW30739	(read/write)	538806	538810	4	0.0
			.app_xip_area	445148	445152	4	0.0
efr32	window-app	BRD4161A	(read only)	816868	816884	16	0.0
			.text	816860	816876	16	0.0
esp32	all-clusters-app	c3devkit	(read only)	933614	933622	8	0.0
			.flash.text	933614	933622	8	0.0
p6	all-clusters-app	default	(read/write)	2442944	2442960	16	0.0
			.text	1401208	1401224	16	0.0
qpg	lock-app	qpg6105+debug	(read only)	518404	518408	4	0.0
			.text	513084	513088	4	0.0
telink	lighting-app	tlsr9518adk80d	(read/write)	848562	848578	16	0.0
			text	593542	593560	18	0.0

Decreases (3 builds for efr32, mbed, qpg)

platform	target	config	section	33c4854	90fca37	change	% change
efr32	lighting-app	BRD4161A	(read only)	844292	844276	-16	-0.0
			.text	844284	844268	-16	-0.0
mbed	shell	CY8CPROTO_062_4343W+release	(read/write)	2294324	2294316	-8	-0.0
			.text	1256896	1256888	-8	-0.0
qpg	lighting-app	qpg6105+debug	(read only)	572284	572280	-4	-0.0
			.text	566964	566960	-4	-0.0

Full report (33 builds for cyw30739, efr32, esp32, k32w, linux, mbed, nrfconnect, p6, qpg, telink)

platform	target	config	section	33c4854	90fca37	change	% change
cyw30739	light	cyw930739m2evb_01	(read/write)	580794	580798	4	0.0
			.app_xip_area	485592	485596	4	0.0
			.bss	77948	77948	0	0.0
			.data	596	596	0	0.0
			.rodata	0	0	0	0.0
			.text	0	0	0	0.0
	lock-app	CYW30739	(read/write)	538806	538810	4	0.0
			.app_xip_area	445148	445152	4	0.0
			.bss	76444	76444	0	0.0
			.data	560	560	0	0.0
			.rodata	0	0	0	0.0
			.text	0	0	0	0.0
efr32	lighting-app	BRD4161A	(read only)	844292	844276	-16	-0.0
			(read/write)	127492	127492	0	0.0
			.bss	125592	125592	0	0.0
			.data	1900	1900	0	0.0
			.text	844284	844268	-16	-0.0
		BRD4161A+rpc	(read only)	831640	831640	0	0.0
			(read/write)	144152	144152	0	0.0
			.bss	142152	142152	0	0.0
			.data	2000	2000	0	0.0
			.text	831632	831632	0	0.0
	window-app	BRD4161A	(read only)	816868	816884	16	0.0
			(read/write)	126148	126148	0	0.0
			.bss	124292	124292	0	0.0
			.data	1856	1856	0	0.0
			.text	816860	816876	16	0.0
esp32	all-clusters-app	c3devkit	(read only)	933614	933622	8	0.0
			(read/write)	1399146	1399146	0	0.0
			.dram0.bss	70984	70984	0	0.0
			.dram0.data	14244	14244	0	0.0
			.flash.rodata	195008	195008	0	0.0
			.flash.text	933614	933622	8	0.0
			.iram0.text	62056	62056	0	0.0
		m5stack	(read only)	981223	981223	0	0.0
			(read/write)	463500	463500	0	0.0
			.dram0.bss	75728	75728	0	0.0
			.dram0.data	34024	34024	0	0.0
			.flash.rodata	221620	221620	0	0.0
			.flash.text	975839	975839	0	0.0
			.iram0.text	123399	123399	0	0.0
k32w	light	k32w061+release	(read/write)	665592	665592	0	0.0
			.bss	77724	77724	0	0.0
			.data	1868	1868	0	0.0
			.text	580200	580200	0	0.0
	lock	k32w061+release	(read/write)	666544	666544	0	0.0
			.bss	77988	77988	0	0.0
			.data	1892	1892	0	0.0
			.text	580864	580864	0	0.0
linux	chip-tool-ipv6only	arm64	(read only)	6765620	6765620	0	0.0
			(read/write)	279201	279201	0	0.0
			.bss	55537	55537	0	0.0
			.data	1128	1128	0	0.0
			.data.rel.ro	175984	175984	0	0.0
			.dynamic	560	560	0	0.0
			.got	42784	42784	0	0.0
			.init	24	24	0	0.0
			.init_array	200	200	0	0.0
			.rodata	386892	386892	0	0.0
			.text	5806628	5806628	0	0.0
	thermostat-no-ble	arm64	(read only)	2083308	2083308	0	0.0
			(read/write)	152129	152129	0	0.0
			.bss	69761	69761	0	0.0
			.data	960	960	0	0.0
			.data.rel.ro	74288	74288	0	0.0
			.dynamic	560	560	0	0.0
			.got	4144	4144	0	0.0
			.init	24	24	0	0.0
			.init_array	336	336	0	0.0
			.rodata	130396	130396	0	0.0
			.text	1735424	1735424	0	0.0
mbed	all-clusters-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2383488	2383488	0	0.0
			.bss	189692	189692	0	0.0
			.data	5288	5288	0	0.0
			.text	1346088	1346088	0	0.0
	lighting-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2341360	2341360	0	0.0
			.bss	181200	181200	0	0.0
			.data	5584	5584	0	0.0
			.text	1303960	1303960	0	0.0
	lock-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2308112	2308112	0	0.0
			.bss	181096	181096	0	0.0
			.data	5568	5568	0	0.0
			.text	1270712	1270712	0	0.0
	pigweed-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	1139712	1139712	0	0.0
			.bss	11756	11756	0	0.0
			.data	4368	4368	0	0.0
			.text	103096	103096	0	0.0
	shell	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2294324	2294316	-8	-0.0
			.bss	177828	177828	0	0.0
			.data	5384	5384	0	0.0
			.text	1256896	1256888	-8	-0.0
nrfconnect	lighting-app	nrf52840dk_nrf52840	(read/write)	984063	984063	0	0.0
			bss	120956	120956	0	0.0
			rodata	116180	116180	0	0.0
			text	669136	669136	0	0.0
		nrf52840dk_nrf52840+rpc	(read/write)	969643	969643	0	0.0
			bss	118000	118000	0	0.0
			rodata	107656	107656	0	0.0
			text	665580	665580	0	0.0
		nrf52840dongle_nrf52840	(read/write)	1000183	1000183	0	0.0
			bss	122128	122128	0	0.0
			rodata	115032	115032	0	0.0
			text	674564	674564	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	893282	893282	0	0.0
			bss	117744	117744	0	0.0
			rodata	109476	109476	0	0.0
			text	585316	585316	0	0.0
	lock-app	nrf52840dk_nrf52840	(read/write)	916675	916675	0	0.0
			bss	119336	119336	0	0.0
			rodata	104816	104816	0	0.0
			text	615120	615120	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	826714	826714	0	0.0
			bss	116152	116152	0	0.0
			rodata	98044	98044	0	0.0
			text	532072	532072	0	0.0
	pigweed-app	nrf52840dk_nrf52840	(read/write)	541779	541779	0	0.0
			bss	52588	52588	0	0.0
			rodata	50048	50048	0	0.0
			text	376940	376940	0	0.0
	pump-app	nrf52840dk_nrf52840	(read/write)	919539	919539	0	0.0
			bss	119088	119088	0	0.0
			rodata	105328	105328	0	0.0
			text	617656	617656	0	0.0
	pump-controller-app	nrf52840dk_nrf52840	(read/write)	914723	914723	0	0.0
			bss	119112	119112	0	0.0
			rodata	104432	104432	0	0.0
			text	613696	613696	0	0.0
	shell	nrf52840dk_nrf52840	(read/write)	798203	798203	0	0.0
			bss	109776	109776	0	0.0
			rodata	78288	78288	0	0.0
			text	533640	533640	0	0.0
p6	all-clusters-app	default	(read/write)	2442944	2442960	16	0.0
			.bss	118036	118036	0	0.0
			.data	2584	2584	0	0.0
			.text	1401208	1401224	16	0.0
	light-app	default	(read/write)	2339856	2339856	0	0.0
			.bss	105780	105780	0	0.0
			.data	2408	2408	0	0.0
			.text	1298120	1298120	0	0.0
	lock-app	default	(read/write)	2305368	2305368	0	0.0
			.bss	105524	105524	0	0.0
			.data	2360	2360	0	0.0
			.text	1263632	1263632	0	0.0
qpg	lighting-app	qpg6105+debug	(read only)	572284	572280	-4	-0.0
			(read/write)	146936	146936	0	0.0
			.bss	89936	89936	0	0.0
			.data	1060	1060	0	0.0
			.text	566964	566960	-4	-0.0
	lock-app	qpg6105+debug	(read only)	518404	518408	4	0.0
			(read/write)	146940	146940	0	0.0
			.bss	89408	89408	0	0.0
			.data	992	992	0	0.0
			.text	513084	513088	4	0.0
	persistent-storage-app	qpg6105+debug	(read only)	107140	107140	0	0.0
			(read/write)	146940	146940	0	0.0
			.bss	38504	38504	0	0.0
			.data	288	288	0	0.0
			.text	101820	101820	0	0.0
telink	lighting-app	tlsr9518adk80d	(read/write)	848562	848578	16	0.0
			bss	87736	87736	0	0.0
			noinit	37160	37160	0	0.0
			text	593542	593560	18	0.0