[Cashtab] Minimize fees for NFT listing txs

Summary:
Cashtab currently requires a user balance of 3000 satoshis to list an NFT. This is bc the way Cashtab build's NFT listing txs -- which require a special "ad prep" tx -- was not fully developed.

Minimize the size of the double tx by ensuring the 2nd tx always has one input. Calculate the required fee of the 2nd tx (the offer tx) so that the 1st tx (the "ad prep" tx) can create an output that is exactly big enough to cover the offer tx with no change.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16864