[GPU] Add vulkan shader disk storage / startup loading

Big refactor of the shader storage to allow both backends to share code

src/xenia/gpu/graphics_system.h

@@ -94,8 +94,9 @@ class GraphicsSystem {
 
   virtual void ClearCaches();
 
-  void InitializeShaderStorage(const std::filesystem::path& cache_root,
-                               uint32_t title_id, bool blocking);
+  void InitializeShaderStorage(
+      const std::filesystem::path& cache_root, uint32_t title_id, bool blocking,
+      std::function<void()> completion_callback = nullptr);
 
   void RequestFrameTrace();
   void BeginTracing();