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/extension.cpp

@@ -30,7 +30,18 @@
  */
 
 #include "extension.h"
-#include <stdlib.h>
+
+#include "swgamedata.h"
+#include "swgameserver.h"
+#include "swhttp.h"
+#include "swhttprequest.h"
+#include "swforwards.h"
+#include "gsnatives.h"
+#include "swgshooks.h"
+#include "swgsdetours.h"
+#include "ssnatives.h"
+#include "swgchooks.h"
+#include "gcnatives.h"
 
 /**
  * @file extension.cpp