[ecash-agora] export partial modules

Summary: Export modules from partial.js so users of the ecash-agora module can use the partial offer features

Test Plan: Confirm this follows how the other features are exported. I had to do this to get it working in Cashtab; it works with this (without it, can't find AgoraPartial)

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D16820