[chronik] Electrum: implement the banner endoint

Summary:
This is useless but is part of the protocol.

Depends on D18151.

Test Plan:

./test/functional/test_runner.py chronik_electrum_basic

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18152