[Cashtab] Do not give a change address to opreturn outputs

Summary: Add opreturn support to new sendxec function. Note: this function is still not implemented in the user-facing GUI of the app.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14949