Fix creating config template using rest api (#13869)

* Fix creation of extras/config-templates objects using the REST API

* Update serializers.py

netbox/extras/api/serializers.py

@@ -454,7 +454,7 @@ class ConfigTemplateSerializer(TaggableModelSerializer, ValidatedModelSerializer
         required=False
     )
     data_file = NestedDataFileSerializer(
-        read_only=True
+        required=False
     )
 
     class Meta: