[mock-chronik-client] Add support for ecash-agora stub mock

Summary:
ecash-agora has been expanded to include some class methods that wrap chronik to create specialized queries (that only work for a server running the ecash-agora chronik plugin).

Add a mock that supports these methods. See D16328 for implementation.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16737