[Cashtab] Add function to get target outputs for NFT1 parent genesis tx

Summary:
Ref D15861 to see this implemented in context.

NFT 1 spec: https://github.com/simpleledger/slp-specifications/blob/master/slp-nft-1.md

Function is not implemented in this diff. Will get supporting functions in one at a time while working through T3528

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15968