Rename chronik-agora to chronik-plugins

Summary:
We may add more plugins at some point, let's not get limited by the name.

Depends on D17492.

Test Plan: \o/

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17493