certAddFuture method
Returns "ok", "Not changed" or {"error": error_message}.
Implementation
Future<MessageOrPromptOrError> certAddFuture(
  String domain,
  String auth_type,
  String auth_user_name,
  String cert,
) async {
  var resultStr = await ZeroNet.instance.cmdFuture(
    ZeroNetCmd.certAdd,
    params: {
      'domain': domain,
      'auth_type': auth_type,
      'auth_user_name': auth_user_name,
      'cert': cert,
    },
  );
  return resultStr.toMsgOrPromptOrErr;
}