feat: tower defense activity

Bare minimum work done.

src/main/java/emu/nebula/server/handlers/HandlerActivityTowerDefenseLevelApplyReq.java

@@ -0,0 +1,17 @@
+package emu.nebula.server.handlers;
+
+import emu.nebula.net.NetHandler;
+import emu.nebula.net.NetMsgId;
+import emu.nebula.net.HandlerId;
+import emu.nebula.net.GameSession;
+
+@HandlerId(NetMsgId.activity_tower_defense_level_apply_req)
+public class HandlerActivityTowerDefenseLevelApplyReq extends NetHandler {
+
+    @Override
+    public byte[] handle(GameSession session, byte[] message) throws Exception {
+        // Encode and send
+        return session.encodeMsg(NetMsgId.activity_tower_defense_level_apply_succeed_ack);
+    }
+
+}

src/main/java/emu/nebula/server/handlers/HandlerActivityTowerDefenseLevelSettleReq.java

@@ -0,0 +1,22 @@
+package emu.nebula.server.handlers;
+
+import emu.nebula.net.NetHandler;
+import emu.nebula.net.NetMsgId;
+import emu.nebula.net.HandlerId;
+import emu.nebula.game.player.PlayerChangeInfo;
+import emu.nebula.net.GameSession;
+
+@HandlerId(NetMsgId.activity_tower_defense_level_settle_req)
+public class HandlerActivityTowerDefenseLevelSettleReq extends NetHandler {
+
+    @Override
+    public byte[] handle(GameSession session, byte[] message) throws Exception {
+        // Initialize change info
+        // TODO: Handle this properly
+        var changeInfo = new PlayerChangeInfo();
+
+        // Encode and send
+        return session.encodeMsg(NetMsgId.activity_tower_defense_level_settle_succeed_ack, changeInfo.toProto());
+    }
+
+}