refactor: Implement generics for `CheckPoint`, `LocalChain`, and `spk_client` types #6971

Job	Run time
prepare	4s
Check WASM	1m 5s
Build & Test (1.84.1, true, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 12s
Check no_std	35s
Build & Test (1.84.1, true, --all-features)	4m 6s
Build & Test Examples (example_cli)	32s
Build & Test (1.63.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	3m 24s
Rust clippy	48s
Build & Test Examples (example_bitcoind_rpc_polling)	37s
Build & Test (1.63.0, --all-features)	4m 27s
Build & Test Examples (example_electrum)	53s
Rust fmt	12s
Build & Test (1.75.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 21s
Build & Test Examples (example_esplora)	55s
Build & Test (1.75.0, --all-features)	3m 57s
Build & Test Examples (example_wallet_electrum)	58s
Build & Test Examples (example_wallet_esplora_async)	1m 6s
Build & Test Examples (example_wallet_esplora_blocking)	56s
Build & Test Examples (example_wallet_rpc)	37s
	29m 45s

Provide feedback