code: func (s *APIServer) getU2FAppID(auth ClientI, w http.ResponseWriter, r *http.Request, p httprouter.Params, version string) (interface{}, error) {
  	cap, err := auth.GetAuthPreference()
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  
  	universalSecondFactor, err := cap.GetU2F()
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  
  	w.Header().Set("Content-Type", "application/fido.trusted-apps+json")
  	return universalSecondFactor.AppID, nil
  }
docs: // getU2FAppID returns the U2F AppID in the auth configuration