[chronik] implement blockchain.transaction.get_merkle Electrum method

Summary:
See https://electrum-cash-protocol.readthedocs.io/en/latest/protocol-methods.html#blockchain-transaction-get-merkle

Depends on D17522

Test Plan:
ninja check-functional

Check the results are identical (same values, same endianness for hashes in the merkle branch) with what Fulcrum returns for a handful of recent main-net transaction.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17514