docs: public AppendObjectResponse appendObject(AppendObjectRequest request) {
          checkNotNull(request, "request should not be null.");
          assertStringNotNullOrEmpty(request.getKey(), "object key should not be null or empty");
  
          InternalRequest internalRequest = this.createRequest(request, HttpMethodName.POST);
          internalRequest.addParameter("append", null);
          if (request.getOffset() != null) {
              internalRequest.addParameter("offset", request.getOffset().toString());
          }
          BosResponse response = uploadObject(request, internalRequest);
          AppendObjectResponse result = new AppendObjectResponse();
          result.setNextAppendOffset(response.getMetadata().getNextAppendOffset());
          result.setContentMd5(response.getMetadata().getContentMd5());
          result.setETag(response.getMetadata().getETag());
          return result;
      }