[Cashtab] Add function to generate an OP_RETURN output for an airdrop tx
Summary:
Per title.
Note: this function is not currently implemented in the app.
Test Plan: npm test
Reviewers: #bitcoin_abc, emack
Reviewed By: #bitcoin_abc, emack
Subscribers: emack
Differential Revision: https://reviews.bitcoinabc.org/D14736