HLE/Services: Allow specifying a SessionData template parameter to ServiceFramework.

Some services can have multiple clients at the same time, and they identify the different clients using the server session as a key.
This parameter (if present) should be a structure that contains the per-session data for each service.
The data can be retrieved using ServiceFramework::GetSessionData(session)

src/core/hle/service/fs/archive.h

@@ -108,6 +108,10 @@ public:
 
 protected:
     void HandleSyncRequest(Kernel::SharedPtr<Kernel::ServerSession> server_session) override;
+
+    std::unique_ptr<SessionDataBase> MakeSessionData() const override {
+        return nullptr;
+    }
 };
 
 /**