[Cashtab] Add new validation function for parsing opreturn as a bip21 parameter

Summary:
Add new function and unit tests for validating a raw hex string to be included in OP_RETURN for an eCash tx. See D15032 for proof of concept / where this is going.

Spec for ref: https://github.com/Bitcoin-ABC/bitcoin-abc/blob/master/doc/standards/bip21-ecash-additions.md

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK, Fabien

Reviewed By: #bitcoin_abc, PiRK, Fabien

Subscribers: PiRK, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15035