[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