docs: public String extractPayloadJsonAsString(String body) {
          if (body == null) {
              return null;
          }
          String[] pairs = body.split("\\&");
          for (String pair : pairs) {
              String[] elements = pair.split("=");
              if (elements.length == 2 && elements[0].equals("payload")) {
                  try {
                      return URLDecoder.decode(elements[1], "UTF-8");
                  } catch (UnsupportedEncodingException e) {
                      return elements[1];
                  }
              }
          }
          return null;
      }