New Dataset mixin for dataset API endpoints

We need a new Dataset model defined in arkindex_worker.models for better manipulation as well as a new dataset module in arkindex_worker/worker.

This module implements a new DatasetMixin just like the other mixin. This mixin will not be inherited by ElementsWorker. The first endpoint we need is list_dataset_elements.

def list_dataset_elements(self, dataset: Dataset) -> Iterator[Tuple[str, Element]]

To keep the paginator, you might try to use map or itertools.starmap.

No cache support here either.