feat!(chain): implement `CheckPointEntry` WIP #7470

Job	Run time
prepare	5s
Rust fmt	15s
Rust clippy	42s
Check no_std	36s
Build & Test (1.88.0, true, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	36s
Build & Test (1.75.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	45s
Build & Test (1.88.0, true, --all-features)	48s
Check WASM	1m 59s
Build & Test (1.63.0, --all-features)	49s
Build & Test (1.75.0, --all-features)	51s
Build & Test (1.63.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	50s
Build & Test Examples (example_cli)	35s
Build & Test Examples (example_bitcoind_rpc_polling)	39s
Build & Test Examples (example_electrum)	49s
Build & Test Examples (example_esplora)	43s
	11m 2s

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