code: func WithCredentials(serverName, ca, cert, key string) ClientOpt {
  	return &withCredentials{serverName, ca, cert, key}
  }
docs: // WithCredentials configures the TLS parameters of the client.
  // Arguments:
  // * serverName: specifies the name of the target server
  // * ca:				 specifies the filepath of the CA certificate to use for verification
  // * cert:			 specifies the filepath of the client certificate
  // * key:				 specifies the filepath of the client key