[tg bot] Support for utf8 memo actions

Summary:
T2972

Depends on D13300

Support parsing memo txs. For now, only support actions where all the text after the action code is parsed to utf8.

Test Plan:
Review new mocks and tests. npm test
npm run mocks
npm run msgs and msgs are parsed, not garbled

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D13318