docs: public ResponseWrapper addCrossFriends(String username, CrossFriendPayload payload)
              throws APIConnectionException, APIRequestException {
          StringUtils.checkUsername(username);
          Preconditions.checkArgument(null != payload, "CrossFriendPayload should not be null");
          return _httpClient.sendPost(_baseUrl + crossUserPath + "/" + username + "/friends", payload.toString());
      }