Adding a new column/sub-field of type entity_reference

Entity reference is not currently supported but I'll work on it. Thanks for bringing to my attention.

The syntax is correct. The original code just wasn't accounting for it. You can try this now with the patch and as long as you have a valid entity type as the target_type, it should process. There is validation in place to ensure a valid target_type is included when data type is entity_reference and will provide messaging in the exception if not.