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`.

CMakeLists.txt

@@ -11,6 +11,7 @@ add_library(uno-game-lib
         src/game/Player.cpp
         src/game/GameState.cpp
         src/common/Utils.cpp
+        src/network/Message.cpp
 )
 
 add_executable(uno-game src/main.cpp)

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

src/network/Message.h

@@ -0,0 +1,60 @@
+/**
+ * @file
+ *
+ * @author Yuzhe Guo
+ * @date 2025.11.18
+ */
+#ifndef UNO_GAME_MESSAGE_H
+#define UNO_GAME_MESSAGE_H
+#include "../game/Card.h"
+#include "../game/CardTile.h"
+#include "../game/Player.h"
+
+
+#include <string>
+#include <variant>
+
+
+namespace UNO::NETWORK {
+    enum class MessagePayloadType { JOIN_GAME, START_GAME, DRAW_CARD, PLAY_CARD, INIT_GAME, END_GAME };
+
+    struct JoinGamePayload {
+        std::string playerName;
+    };
+
+    struct StartGamePayload {};
+
+    struct DrawCardPayload {
+        int drawCount;
+    };
+
+    struct PlayCardPayload {
+        GAME::Card card;
+    };
+
+    struct InitGamePayload {
+        GAME::DiscardPile discardPile;
+        GAME::HandCard handCard;
+        size_t currentPlayerIndex;
+    };
+
+    struct EndGamePayload {};
+
+    using MessagePayload =
+        std::variant<std::monostate, JoinGamePayload, StartGamePayload, DrawCardPayload, PlayCardPayload, InitGamePayload, EndGamePayload>;
+
+    class Message {
+    private:
+        MessagePayloadType messagePayloadType_;
+        MessagePayload messagePayload_;
+
+    public:
+        Message(MessagePayloadType messagePayloadType, MessagePayload messagePayload);
+
+        [[nodiscard]] MessagePayloadType getMessagePayloadType() const;
+        [[nodiscard]] MessagePayload getMessagePayload() const;
+    };
+
+}   // namespace UNO::NETWORK
+
+#endif   // UNO_GAME_MESSAGE_H