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)