Fix circular includes and include cleanup (#3)

* Fix circular includes and include cleanup

Fixed a bug where other *.h header files would include extension.h, which itself
was already included in those same headers, creating a circular dependency cycle.
This caused build errors and made compilation unpredictable.

To resolve this:
- Moved implementation-only includes to .cpp files where they are actually needed.
- Added forward declarations where possible to avoid pulling in unnecessary headers.
- Ensured extension.h only contains declarations that require full type definitions.

This improves build stability and compilation time.

* Clean up includes and fix build errors

- Replaced full includes with forward declarations where possible
- Moved implementation-only includes from .h to .cpp files
- Added missing includes for stdint.h and Steam interfaces
- This significantly reduces compilation time and prevents future dependency issues

* Switch to SourceMod SDK smsdk_ext instead of local copy

Extension/swgamedata.h

@@ -17,7 +17,11 @@
 */
 
 #pragma once
-#include "smsdk_ext.h"
+
+namespace SourceMod
+{
+	class IGameConfig;
+}
 
 class SteamWorksGameData
 {
@@ -27,10 +31,8 @@ class SteamWorksGameData
 
 	public:
 		bool HasGameData(void) const;
-		IGameConfig *GetGameData(void) const;
+		SourceMod::IGameConfig *GetGameData(void) const;
 
 	private:
-		IGameConfig *pGameConf;
+		SourceMod::IGameConfig *pGameConf;
 };
-
-#include "extension.h"