[chronik] Electrum: Add the address.get_scripthash endpoint

Summary: This adds initial support for the various blockchain.address.* endpoints. For now only get_scripthash is supported and only cash address with a prefix are compatible.

Test Plan:

./test/functional/test_runner.py chronik_electrum_blockchain

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18118