[Cashtab] Create targetOutput for a Cashtab Msg tx

Summary:
To support refactor of tx creation and broadcasting functions using ecash-coinselect, create a new opreturn folder initialized with function to create desired targetOutput for a Cashtab Msg tx

Existing opreturn output generation is part of overloaded tx creation functions. Each opreturn type should have its own function.

This diff does not implement the function or change any app functionality of Cashtab.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14735