[ecash-agora] Partial Agora offers: Add approximation logic

Summary:
Add AgoraPartial and AgoraPartialParams for offers of fungible tokens, and add (only) the script param approximation logic.

This is rather complex, and is explained in the comments. The corresponding Script can be seen in D16644, which will be added in later diffs.

Test Plan: npm run integration-tests

Reviewers: #bitcoin_abc, bytesofman, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16735