code: func postCallback(opts *ServerOptions) Handler {
  	return func(_ io.Writer, r io.Reader, _ httprouter.Params) error {
  		req := &CallbackRequest{}
  		err := json.NewDecoder(r).Decode(req)
  		if err != nil {
  			return err
  		}
  		return opts.Controller.Callback(req)
  	}
  }
docs: // opengdpr_callbacks