[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