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/AMBuilder

@@ -60,8 +60,8 @@ project.sources += [
   'swhttp.cpp',
   'swhttprequest.cpp',
   'swgchooks.cpp',
-  'sdk/smsdk_ext.cpp',
   os.path.join(SteamWorks.sm_root, 'public', 'CDetour', 'detours.cpp'),
+  os.path.join(SteamWorks.sm_root, 'public', 'smsdk_ext.cpp'),
 ]
 
 binary = SteamWorks.HL2Config(project, builder.cxx, 'SteamWorks.ext')