[Cashtab] Add function to generate burn targetOutputs

Summary:
Add function intended to deprecate cashMethods generateBurnOpReturn as part of transaction construction refactors

This diff does not yet implement the function

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15100