Changes to Add a custom variable to an existing model

the response property user_id became optional for the status 201

response property user_id list-of-types was widened by adding types null to media type application/json of response 201

added the optional property project_id to the response with the 201 status

added the optional property source_model_id to the response with the 201 status

for the path request parameter model_id, the format changed from no format to uuid