[mock-chronik-client] Support sub/unsub address method for ws
Summary:
Ref D15611, support this new chronik-client functionality in mock-chronik-client
Depends on D15621
Test Plan: npm test
Reviewers: #bitcoin_abc, emack
Reviewed By: #bitcoin_abc, emack
Subscribers: emack
Differential Revision: https://reviews.bitcoinabc.org/D15613