Use structs for config callback IDs

This way you can't mix up regular config callback IDs and CPU thread
config callback IDs. (It would be rather bad if you did!)

Source/Core/InputCommon/GCAdapter.cpp

@@ -23,6 +23,7 @@
 #endif
 
 #include "Common/BitUtils.h"
+#include "Common/Config/Config.h"
 #include "Common/Event.h"
 #include "Common/Flag.h"
 #include "Common/Logging/Log.h"
@@ -158,7 +159,7 @@ static u8 s_endpoint_out = 0;
 
 static u64 s_last_init = 0;
 
-static std::optional<size_t> s_config_callback_id = std::nullopt;
+static std::optional<Config::ConfigChangedCallbackID> s_config_callback_id = std::nullopt;
 
 static bool s_is_adapter_wanted = false;
 static std::array<bool, SerialInterface::MAX_SI_CHANNELS> s_config_rumble_enabled{};