[Cashtab] Generate targetOutputs for SLP genesis tx
Summary:
Add new function to generate a targetOutput for an SLP genesis tx.
This diff adds and unit tests the function. Function is not implemented.
Goal is to deprecate generateGenesisOpReturn and generateTokenTxOutput functions in cashMethods.js
Test Plan: npm test
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D15060