[Cashtab] Add unimplemented functions to support NFT minting and sending

Summary:
Add functions Cashtab will need in order to support minting and sending NFTs.

Integration tests will be added for these functions on implementation.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16001