[chronik] implement the blockchain.scripthash.get_balance Electrum method

Summary:
See https://electrum-cash-protocol.readthedocs.io/en/latest/protocol-methods.html#blockchain.scripthash.get_balance

Depends on D17611

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17605