[ecash-agora] Partial Agora offers: Add ALP support to plugin, Agora and AgoraOffer

Summary:
In D16745 we added support for SLP, in this diff we add support for ALP.

The biggest difference is that offers use the OP_RETURN to advertize instead of the scriptSig like in SLP.

Depends on D16754.

Test Plan: npm run integration-tests

Reviewers: bytesofman, #bitcoin_abc, Fabien

Reviewed By: bytesofman, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16755