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/swhttp.h

@@ -17,9 +17,8 @@
 */
 
 #pragma once
-#include "isteamgameserver.h"
-#include "steam_gameserver.h"
-#include "smsdk_ext.h"
+
+#include <smsdk_ext.h>
 
 #include <unordered_map>
 
@@ -57,6 +56,3 @@ class SteamWorksHTTP :
 		HandleType_t typeHTTP;
 		std::unordered_map<HTTPRequestHandle, SteamWorksHTTPRequest *> m_Requests;
 };
-
-#include "swhttprequest.h"
-#include "extension.h"