refactor: Implement generics for `CheckPoint`, `LocalChain`, and `spk_client` types #6559

Job	Run time
prepare	2s
Check WASM	1m 5s
Check no_std	35s
Build & Test Examples (example_cli)	29s
Build and test (1.83.0, true, --no-default-features --features miniscript/no-std)	2m 20s
Build & Test Examples (example_bitcoind_rpc_polling)	37s
clippy_check	1m 13s
Rust fmt	15s
Build and test (1.83.0, true, --all-features)	3m 53s
Build & Test Examples (example_electrum)	56s
Build & Test Examples (example_esplora)	56s
Build and test (1.63.0, --no-default-features --features miniscript/no-std)	3m 37s
Build & Test Examples (example_wallet_electrum)	53s
Build and test (1.63.0, --all-features)	5m 6s
Build & Test Examples (example_wallet_esplora_async)	56s
Build & Test Examples (example_wallet_esplora_blocking)	56s
Build & Test Examples (example_wallet_rpc)	42s
	24m 31s

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