[ecash-agora] Move helper functions to get utxos for accepting and canceling agora txs from Cashtab to ecash-agora

Summary:
ecash-agora exports helpful methods for getting the tx fee for accept or cancel txs. Also the AgoraOffer object has methods to accept or cancel. However, these methods require fuel utxos. Right now, it's on the app dev to get those utxos.

These functions belong in the libray, since

• chronik-client is already a dependency of ecash-agora
• utxos are typed, so we can expect app devs to be working with chronik-client utxos
• they are specifically used for accepting or canceling agora txs

Move functions and tests from Cashtab to ecash-agora lib. Export from the lib and implement in Cashtab to show they still work as expected.

Test Plan: npm test, CI tests

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17637