public User PostLogin([FromBody] UsernameRequest req)
        {
            User user = new User(req.username);

            onlineUsers.Add(user);
            return(user);
        }
 public UsersLogoutResponse PostLogout([FromBody] UsernameRequest req)
 {
     onlineUsers.Remove(new User(req.username));
     return(new UsersLogoutResponse(true));
 }