List Custom Fieldsets

{
  "items": [
    {
      "id": "string",
      "entity_type": "user",
      "name": "string",
      "fields": [
        {
          "id": "string",
          "field_definition_id": "string",
          "display_order": 0,
          "required_override": true,
          "hidden_override": true
        }
      ],
      "created_at": null,
      "updated_at": null
    }
  ],
  "total": 0,
  "page": 1,
  "size": 1,
  "pages": 0
}

{
  "detail": [
    {
      "loc": [
        "string"
      ],
      "msg": "string",
      "type": "string",
      "input": null,
      "ctx": {}
    }
  ]
}