static ClientAuth getClientAuth(SSLConfig config) {
    switch (config.sslClientAuthentication) {
      case "required":
        return ClientAuth.REQUIRE;
      case "requested":
        return ClientAuth.OPTIONAL;
      default:
        return ClientAuth.NONE;
    }
  }
@param config the {@link SSLConfig}.
@return the {@link ClientAuth} setting.
ambry-commons/src/main/java/com/github/ambry/commons/NettySslFactory.java