Create a model with field definitions in the same request

(Copied from Create a model with fields?)

When creating a new model, it’d be nice to be able to specify the field definitions at the same time, in the same request. Something like (example format borrowed from the API response when you query a model):

{
  "data": {
    "type": "item_type",
    "attributes": {
      "name": "A new model",
      "api_key": "new_model"
    },
    "relationships": {
      "fields": {
        "data": [
          {
            "type": "field",
            "field_type": "string",
            "attributes": {
              "label": "title",
              "appearance": {
                "editor": "single_line",
                "parameters": {
                  "heading": true
                },
                "type": "title"
              }
            }
          },
          {
            "type": "field",
            "field_type": "date_time",
            "attributes": {
              "label": "Date and time",
              "appearance": {
                "editor": "date_time_picker"
              }
            }
          }
        ]
      }
    }
  }
}

This would allow easier programmatic creation (or duplication) of models, rather than having to loop through each field one at a time.