Gremlin-JavaScript: Support SASL authentication

/** @abstract */
class Authenticator {
  /**
  @returns {Promise} Returns a promise containing the Request to be sent.
  When the Promise resolves to a null value, it indicates that the authentication flow was successful.
  */ 
  evaluateChallenge(response) {
    throw new Error("evaluateChallenge should be implemented");
  }
}