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/swhttprequest.h

@@ -17,9 +17,9 @@
 */
 
 #pragma once
-#include "isteamgameserver.h"
-#include "steam_gameserver.h"
-#include "smsdk_ext.h"
+
+#include <isteamhttp.h>
+#include <smsdk_ext.h> // HTTPRequestHandle, Handle_t, IChangeableForward
 
 class SteamWorksHTTPRequest
 {
@@ -43,9 +43,9 @@ class SteamWorksHTTPRequest
 		CCallResult<SteamWorksHTTPRequest, HTTPRequestCompleted_t> CompletedCallResult;
 
 	public:
-		IChangeableForward *pCompletedForward;
-		IChangeableForward *pHeadersReceivedForward;
-		IChangeableForward *pDataReceivedForward;
+		SourceMod::IChangeableForward *pCompletedForward;
+		SourceMod::IChangeableForward *pHeadersReceivedForward;
+		SourceMod::IChangeableForward *pDataReceivedForward;
 };
 
 class SteamWorksHTTPNatives
@@ -54,5 +54,3 @@ class SteamWorksHTTPNatives
 		SteamWorksHTTPNatives();
 		~SteamWorksHTTPNatives();
 };
-
-#include "extension.h"