[Cashtab] Add function to create targetOutput from opreturn as param input

Summary:
Follow model of existing functions that create a desired targetOutput for use in sendXec function

This diff does not implement the function. Ref proof of concept D15032

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15082