Add worker_run_id field to ElementListSerializer

Necessary for https://gitlab.com/teklia/arkindex/frontend/-/issues/1189; no information on the worker run is currently returned when listing element children, only worker_version_id.

• Do we also want to be able to filter by worker_run?
• If yes, currently when worker_version filter is set to False it only returns elements created by humans; same thing for worker runs presumably?
• The worker_version and worker_run filters are mutually exclusive?