feat: 实时在线用户统计

src/main/java/com/youlai/system/service/WebsocketService.java

@@ -0,0 +1,10 @@
+package com.youlai.system.service;
+
+public interface WebsocketService {
+
+   void addUser(String username);
+
+   void removeUser(String username) ;
+
+   int getOnlineUserCount() ;
+}