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

Summary:
In D16743 script was added, and now we make it usable by both indexing these offers via the agora.py plugin, as well as adding support to accepting/cancelling them via the Agora class, and AgoraOffer, respectively.

Depends on D16743.

Test Plan: npm run integration-tests

Reviewers: bytesofman, #bitcoin_abc, Fabien

Reviewed By: bytesofman, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16745