Add median-time-past (MTP) calculation to CheckPoint #7543

Job	Run time
prepare	5s
Rust fmt	17s
Rust clippy	1m 18s
Check no_std	39s
Build & Test (1.88.0, true, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 13s
Build & Test (1.88.0, true, --all-features)	3m 40s
Build & Test (1.63.0, --all-features)	5m 46s
Build & Test (1.75.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 23s
Build & Test (1.75.0, --all-features)	4m 8s
Build & Test Examples (example_bitcoind_rpc_polling)	44s
Build & Test Examples (example_cli)	34s
Check WASM	1m 25s
Build & Test Examples (example_esplora)	53s
Build & Test (1.63.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	4m 26s
Build & Test Examples (example_electrum)	1m 4s
	29m 35s

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