refactor: Implement generics for `CheckPoint`, `LocalChain`, and `spk_client` types #6045

Job	Run time
Rust fmt	4s
Check no_std	37s
Build and test (stable, true, --no-default-features)	3m 59s
Check WASM	49s
Build and test (stable, true, --all-features)	3m 56s
prepare	5s
Build and test (1.63.0, --no-default-features)	5m 13s
Build and test (1.63.0, --all-features)	5m 17s
Build Examples (example_cli)	32s
Build Examples (example_bitcoind_rpc_polling)	39s
Build Examples (example_electrum)	53s
Build Examples (example_esplora)	53s
Build Examples (wallet_electrum)	46s
Build Examples (wallet_esplora_async)	1m 0s
Build Examples (wallet_esplora_blocking)	54s
Build Examples (wallet_rpc)	43s
clippy_check	1m 18s
	27m 38s

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