[apps and Cashtab] Implement monorepo chronik mock and deprecate repo-specific implementations

Summary:
Mocked chronik client was created initially in alias-server to support testing there. Later it was ported to other repos that needed it.

Plan is to get it into chronik-client. For now, though, it should at least live in only one place, so that it isn't maintained many times in the same monorepo.

Implementing.

Test Plan: npm test in apps/ecash-herald, apps/alias-server, and cashtab

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14518