Add `blockchain::GetBlockHash` trait and implement for blockchains

[notmandatory]

The new trait should have a function to get a block hash at a given height. The Esplora API, Electrum API, RPC, and CBF blockchains should all support getting a block hash at a given height. This feature is needed to help support wallets that also use LDK, See https://github.com/bitcoindevkit/bdk-swift/issues/10.

[vladimirfomene]

Hello @notmandatory , can I take this one up?

[notmandatory]

Here's the LDK API we're working with: https://lightningdevkit.org/tutorials/build_a_node_in_java/#11-initialize-the-channelmanager

[thunderbiscuit]

And just for reference on the Rust side.