code: func UserJWT(userCB UserJWTHandler, sigCB SignatureHandler) Option {
  	return func(o *Options) error {
  		if userCB == nil {
  			return ErrNoUserCB
  		}
  		if sigCB == nil {
  			return ErrUserButNoSigCB
  		}
  		o.UserJWT = userCB
  		o.SignatureCB = sigCB
  		return nil
  	}
  }
docs: // UserJWT will set the callbacks to retrieve the user's JWT and
  // the signature callback to sign the server nonce. This an the Nkey
  // option are mutually exclusive.