[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