[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