[Chronik] Add /plugin/:plugin_name/:group_hex/utxos HTTP endpoint

Summary:
Group UTXOs by the groups assigned by plugins using the generic MempoolGroupUtxos and GroupUtxoWriter structs.

Allows plugins to make it easy to find certain kinds of UTXOs, for example, an Agora plugin can color UTXOs by token ID such that we can get all live Agora offers with that token ID.

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

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16541