[mock-chronik-client] Support in-node websocket subscription methods

Summary:
Need this to migrate Cashtab to in-node chronik-client.

Add support for chronik-client methods as they exist in ChronikClientNode

Test Plan: npm test, CI to ensure other tests are not broken

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15694