[Cashtab] [Chronik Websocket Notifications] [Part 8] Check cache for eToken info

Summary:
T2447

Depends on D11538

Add a helper function to check cache for token info required to parse an incoming eToken transaction.

Test Plan: npm test for new unit tests

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D11539