The optional EventSplit expands input_event into child events before filtering and transformation.
Every child receives the authoritative source write key.

The event to filter and transform. It may be an original client event or intermediate output;
provide original_event when source metadata is no longer present. EventSplit expands this event.

The optional unmodified client event supplies metadata removed from input_event. It provides
the playbook event name for a single transform and the authoritative write key when splitting.

The optional filter runs against input_event before the playbook. When input_event is split,
the filter runs independently against each child.

The optional playbook transforms input_event after pipeline filtering. At least one of
playbook, pipeline_filter, or event_split is required. Split children are transformed independently.