[ecash-agora, ecash-wallet, ecash-lib] Add a new take method to the AgoraOffer class that automatically builds and broadcasts an acceptTx

Summary:
ecash-agora is a powerful library that will allow transaction building to accept agora offers. However, since there is no wallet logic in ecash-agora, there is some trickiness with passing params and determining utxos.

It makes sense for ecash-wallet to "just handle" agora stuff. However, it does not really make sense to import the whole ecash-agora lib into ecash-wallet to solve this problem.

Follow the existing API of the acceptTx method (which builds a tx given user-determined wallet inputs), and add a new take method that does this automatically by passing a Wallet.

Test Plan: npm test, CI for integration tests

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18833