docs: public Signature createAppSignature(String prepayId) {
          String pack = "Sign=WXPay";
  
          long timestamp = System.currentTimeMillis() / 1000;
          String nonce = RandomStringGenerator.getRandomStringByLength(16);
          String appId = paySetting.getAppId();
          String mchId = paySetting.getMchId();
          String key = paySetting.getKey();
  
          String sortString = String.format("appId=%s&nonceStr=%s&package=%s&partnerid=%s&&prepayid=%s&timeStamp=%s",
                  appId, nonce, pack, mchId, prepayId, timestamp);
  
          return createSignature(pack, timestamp, nonce, appId, key, sortString);
      }