[tg bot] Parse OP_RETURN msgs

Summary:
T2972

Depends on D13297

Parse OP_RETURN msgs

Test Plan:
npm test
npm run msgs and OP_RETURN msgs are parsed, txs link to them

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D13298