New endpoint to list elements in a dataset

Refs https://redmine.teklia.com/issues/3653

This endpoint will be used by workers to initiate work on dataset elements.

They'll need extensive information for each element:

• the set it belongs to (train, test, val, ...)
• element
  • name
  • type ID and slug
  • polygon
  • ID
  • IIIF image url with bounding box

If possible, resuse an existing "light" serializer, otherwise create a new one. The data can on several level

[
  {
    "set": "train",
    "element": {
       "id": ...,
       "name": ...,
       "type": {"id": ..., "slug": ...},
       "polygon": ...,
       "image": ...,
    }
  }
]

As the workers that will use that API are not created fully yet, you can create the serializers freely: you won't break anything.