Make worker activities mandatory on Workers processes

Remove the worker_activity field from StartProcess, and replace it with a HiddenField named activity_state that defaults to ActivityState.Pending, so that worker activities are always marked as pending when creating processes with this endpoint.
Update the StartProcess unit tests so that they properly mock initialize_activity.delay.
Make sure at least one test is still sending "worker_activity": False, and that it has no effect at all; worker activities will still be used.