[ecash-agora] Add a cancel method to AgoraOffer that can use ecash-wallet to cancel a partial or oneshot offer

Summary: Continue bringing ecash-wallet in line with required features for Cashtab. We do not strictly need this, we could keep using cancelTx, but it is much easier to leverage a Wallet for this use case.

Test Plan: npm test, CI for integration tests

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18842