Registers an external MLflow observer that will receive a
`register_tracking_event(event_name, data)` callback on any model tracking
event such as "create_run", "delete_run", or "log_metric".
Each observer should have a `register_tracking_event(event_name, data)`
callback accepting a character vector `event_name` specifying the name of
the tracking event, and `data` containing a list of attributes of the event.
The callback should be non-blocking, and ideally should complete
instantaneously. Any exception thrown from the callback will be ignored.