Support creating Dataset processes in CreateProcess

Depends on #1548 (closed)

Step 3 of https://redmine.teklia.com/issues/3653: CreateProcess needs a new mode field that allows to select only between a Workers and a Dataset process mode. It must default to Workers to prevent a breaking change to the API.

The selection field, which causes the endpoint to create ProcessElement instances immediately, should not be allowed when using the Dataset mode.