[chronik] Electrum: implement address.get_balance, get_history and listunspent
Summary:
This is just a wrapper around the scripthash.* equivalent.
Depends on D18152.
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/D18153