[DEPRECATED] Creates a Custom Field

curl --request POST \
  --url https://api.rootly.com/v1/custom_fields \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/vnd.api+json' \
  --data '
{
  "data": {
    "type": "custom_fields",
    "attributes": {
      "label": "<string>",
      "description": "<string>",
      "shown": [],
      "required": [],
      "default": "<string>",
      "position": 123
    }
  }
}
'

{
  "data": {
    "id": "<string>",
    "type": "custom_fields",
    "attributes": {
      "label": "<string>",
      "shown": [],
      "required": [],
      "position": 123,
      "created_at": "<string>",
      "updated_at": "<string>",
      "kind": "<string>",
      "enabled": true,
      "slug": "<string>",
      "description": "<string>",
      "default": "<string>"
    }
  },
  "included": [
    {
      "id": "<string>",
      "type": "<string>",
      "attributes": {},
      "relationships": {}
    }
  ]
}