[chronik] implement mock server.peers.subscribe electrum RPC

Summary:
For now we don't implement peering and we just return an empty server list.
Note that this method is poorly named, it is not a subscription.

See doc: https://electrum-cash-protocol.readthedocs.io/en/latest/protocol-methods.html#server.peers.subscribe

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18111