List product variant fields

curl --request GET \
  --url https://api.shoppex.io/dev/v1/products/{id}/variants/fields \
  --header 'Authorization: Bearer <token>'

{
  "data": [
    {
      "id": "field_1",
      "name": "Region",
      "type": "select",
      "required": true,
      "placeholder": null,
      "default_value": null,
      "sort_order": 0,
      "options": [
        {
          "id": "option_1",
          "label": "Europe",
          "value": "eu",
          "sort_order": 0,
          "created_at": 1711953600,
          "updated_at": 1711953900
        }
      ],
      "created_at": 1711953600,
      "updated_at": 1711953900
    }
  ],
  "pagination": {
    "next_cursor": null,
    "has_more": false
  }
}