feat(network): add Message class with payload types

- Introduced `Message` class in `src/network` for handling game-related message payloads (e.g., `JOIN_GAME`, `START_GAME`, etc.).
- Added `Message.cpp` and `Message.h` to `CMakeLists.txt`.

src/network/Message.cpp

@@ -0,0 +1,28 @@
+/**
+ * @file
+ *
+ * @author Yuzhe Guo
+ * @date 2025.11.18
+ */
+#include "Message.h"
+
+#include <utility>
+
+namespace UNO::NETWORK {
+    Message::Message(MessagePayloadType messagePayloadType, MessagePayload messagePayload) :
+        messagePayloadType_(messagePayloadType), messagePayload_(std::move(messagePayload))
+    {
+    }
+
+    MessagePayloadType Message::getMessagePayloadType() const
+    {
+        return this->messagePayloadType_;
+    }
+
+    MessagePayload Message::getMessagePayload() const
+    {
+        return this->messagePayload_;
+    }
+
+
+}   // namespace UNO::NETWORK