[mock-chronik-client] Add subscribeToBlocks and isSubscribedBlocks to match in-node chronik-client

Summary: Used in D15586 and could be used by future apps working with in-node chronik-client

Test Plan: npm test, also tests of alias-server, ecash-herald, and cashtab still pass (tested locally)

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15587