fetchUserInfo method
根据accountId,远程拉取用户信息
Implementation
@override
Future<List<NIMUserInfo>?> fetchUserInfo(List<String> accountList) async {
  var res = await NimCore.instance.userService.getUserList(accountList);
  if (res.isSuccess && res.data != null) {
    return res.data;
  }
  return null;
}