fix model_configurations post request param type

[Jami159]

Was getting the following error when making a post request to http://localhost:8080/api/model_configurations:

{
    "timestamp": "2023-10-04T18:34:02.816+00:00",
    "status": 500,
    "error": "Internal Server Error",
    "trace": "java.lang.IllegalArgumentException: No serializer found for class java.lang.Object and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS)...",
    "message": "No serializer found for class java.lang.Object and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS)",
    "path": "/model_configurations"
}

Specifying the param type fixed it for me.