Add optional defmt support #3564

Job	Run time
alternative_targets (ubuntu-latest, wasm32-unknown-emscripten)	19s
features_check (ubuntu-latest)	6m 31s
alternative_targets (ubuntu-latest, aarch64-apple-ios)	13s
no_std (ubuntu-latest, thumbv6m-none-eabi)	47s
rust_msrv (ubuntu-latest)	43s
alternative_targets (ubuntu-latest, aarch64-linux-android)	26s
no_std (ubuntu-latest, x86_64-fortanix-unknown-sgx)	19s
rust_versions (ubuntu-latest, stable)	1m 27s
rust_versions (ubuntu-latest, beta)	1m 3s
test_wasm (ubuntu-latest)	28s
timezones (ubuntu-latest, ACST-9:30)	1m 20s
rust_versions (ubuntu-latest, nightly)	1m 12s
timezones (ubuntu-latest, EST4)	1m 15s
timezones (ubuntu-latest, UTC0)	1m 8s
check-docs	20s
cross-targets (x86_64-unknown-illumos)	1m 9s
timezones (ubuntu-latest, Asia/Katmandu)	1m 24s
timezones (macos-latest, ACST-9:30)	34s
cross-tests (ubuntu-latest)	1m 37s
test_wasip1 (ubuntu-latest, wasm32-wasip1)	12s
timezones (macos-latest, EST4)	40s
timezones (macos-latest, UTC0)	32s
timezones (macos-latest, Asia/Katmandu)	28s
timezones (windows-latest, ACST-9:30)	2m 12s
timezones (windows-latest, EST4)	2m 27s
timezones (windows-latest, UTC0)	2m 6s
timezones (windows-latest, Asia/Katmandu)	2m 24s
	33m 16s

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