[Chronik] Rename group in PluginOutput to groups

Summary:
Everywhere else in the code this is called groups (e.g. in the protobuf, and in PluginOutputEntry in tx.py), so it's inconsistent to have it be called group for the same thing.

This is a breaking change, but since plugins are still experimental and very few people are using them, it's better to do this change now than later.

Test Plan: ./test/functional/test_runner.py chronik_plugins chronik_plugin_groups

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16674