code: public static Single<String> read(String path) {
          return SingleRxXian.call("cosService", "cosRead", new JSONObject() {{
              put("path", path);
          }}).flatMap(response -> {
              response.throwExceptionIfNotSuccess();
              return Single.just(Objects.requireNonNull(response.dataToStr()));
          });
      }
docs: 从云端读取文件
  
  @param path 文件相对路径
  @return 文件内容