group_name missing from CustomObjectTypeFieldSerializer

### Plugin Version

0.4.0

### NetBox Version

4.4.4

### Python Version

3.12

### Steps to Reproduce

Access http://127.0.0.1:8000/api/plugins/custom-objects/custom-object-type-fields/


### Expected Behavior

Field should be added.

### Observed Behavior

`CustomObjectTypeFieldSerializer` is missing `group_name` from `fields`, meaning that field is not returned in API results.