The purpose of this endpoint is to allow users an easy way to explore the groups indexed by a plugin. These APIs are designed to be extremely fast, by basically slicing exactly the requested info from the DB without having to search through the DB in any way.
The `prefix` param is intended to make plugins prefix different kinds of groups with different prefixes (e.g. "T" for token IDs, and "P" for public keys, etc.), so this allows a simple selection of "sub-groups" of a plugin.
The `start` params is intended to allow a form of pagination; users can put the last group of the last query here to get the next page.
An example how this endpoint would be useful is for the Agora plugin:
1. It allows the user to get every token ID that has an offer, so they can have an overview what tokens even exists.
2. Then, when the user clicks on a token ID, the app queries all the offered UTXOs for this token ID and can display them.
Depends on D16551.