code: func (s *AuthServer) PreAuthenticatedSignIn(user string) (services.WebSession, error) {
  	sess, err := s.NewWebSession(user)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  	if err := s.UpsertWebSession(user, sess); err != nil {
  		return nil, trace.Wrap(err)
  	}
  	return sess.WithoutSecrets(), nil
  }
docs: // PreAuthenticatedSignIn is for 2-way authentication methods like U2F where the password is
  // already checked before issuing the second factor challenge