[Cashtab] Support op_return_raw in SendXec

Summary:
Depends on D15206

Implement support for the op_return_raw bip21 param in Cashtab.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15209