[chronik] Electrum: implement peer discovery endpoints

Summary:
This adds the server.add_peer endpoint and use this data to return in peers.subscribe.

Peer validation is not implemented yet, which means that no added peers will end up in the subscribe returned list for now.

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/D18151