Enable plugins to hook server message response

* Add message handler so that the plugin can hook inside the `dropMessage` method for `Player` instance.

src/main/java/emu/grasscutter/utils/MessageHandler.java

@@ -0,0 +1,21 @@
+package emu.grasscutter.utils;
+
+public class MessageHandler {
+    private String message;
+
+    public MessageHandler(){
+        this.message = "";
+    }
+
+    public void append(String message){
+        this.message += message;
+    }
+
+    public String getMessage(){
+        return this.message;
+    }
+
+    public void setMessage(String message){
+        this.message = message;
+    }
+}