[ecash-agora] Add a helper function to prepare agora listings for ecash-wallet

Summary:
Add new getAgoraPaymentAction function to ecash-agora, to prepare a payment.Action that ecash-wallet can use to broadcast a valid agora tx.

For now, we only support ALP AgoraPartial 'LIST' actions. But we add and test error handling for other actions, so that we have an easy way to extend functionality.

Looked at adding this kind of support directly into ecash-wallet, but I think this kind of approach is best for preserving the modularity / optionality of agora features while also allowing full agora support in ecash-wallet.

Test Plan: npm test, CI integration tests

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18673