Restrict Dataset state updates to Ponos tasks on Dataset processes

Refs https://redmine.teklia.com/issues/3653#change-27167

Updating a dataset's state must only be allowed through Ponos task authentication, for a task on a Dataset process where the dataset is part of the process' datasets.

The state can only change according to this state machine:

Other state changes must be forbidden.

Setting the state to complete should cause Dataset.task_id to be set to the currently authenticated task, to mark it as the one that generated the dataset's artifacts.

The API documentation should be updated to document these authentication requirements, permitted state changes and the automatic task_id update.