Create new endpoint ListCorpusEntities

Goal 1: be able to list all entities in a corpus

Goal 2: be able to list all entities in a specified "folder" in said corpus (through an optional parameter)

This could be several Merge requests.

Do NOT support entities through metadata here, only entities through transcriptions.

Refs https://gitlab.com/arkindex/requests/-/issues/188 & https://gitlab.com/arkindex/requests/-/issues/189