docs: public String fetch(String code) {
          String url = WxEndpoint.get("url.template.get");
          String json = String.format("{\"template_id_short\":\"%s\"}", code);
  
          logger.debug("template message, get template id: {}", json);
          String response = wxClient.post(url, json);
          Map<String, Object> result = JsonMapper.defaultMapper().json2Map(response);
  
          if(result.containsKey("template_id")) {
              return result.get("template_id").toString();
          } else {
              throw new WxRuntimeException(999, "fetch template id failed.");
          }
      }