Fixes: #224 - Use a ContentTypeField in CustomObjectTypeFieldSerializer

[bctiemann]

Fixes: #224

Aligns the CustomObjectTypeFieldSerializer with the behavior of CustomFieldSerializer by allowing the user to specify a related_object_type in the POST body rather than app_label and model fields:

{
    "custom_object_type": 26,
    "type": "multiobject",
    "name": "test2",
    "default": [14],
    "related_object_type": "dcim.device"
}

[bctiemann]

This is technically a breaking change, but since #224 is characterized (correctly) as a bug this is really just aligning to "correct" (core) behavior by using the NetBox-standard ContentTypeField.