Allow RetrieveWorkerRun to Ponos agents on assigned processes
Closes #1601 (closed). I restricted the endpoint to only processes that have tasks assigned to the agent, since in the context of this issue it makes no sense for the agent to retrieve the WorkerRun of anything not assigned to itself. We could tighten this further with a Task.worker_run
foreign key, the same as is already required for #1454 (closed).