[Cashtab] Convert CashtabCache to ts

Summary:
Part of incremental conversion of Cashtab to ts

Will use these types in our first ts component, Collection, for NFTs.

Keep it in a limited diff.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17078