docs: public static void signParams(Map<String, String> params) {
  
      String requestString =
          params.entrySet().stream()
              .sorted(Comparator.comparing(Map.Entry::getKey))
              .map(e -> e.getKey() + "=" + e.getValue())
              .map(String::toUpperCase)
              .collect(Collectors.joining("&"));
  
      String sign = md5(requestString).toLowerCase();
  
      params.put("sign", sign);
      params.remove("secret");
    }