[GPU] Add vulkan shader disk storage / startup loading

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

src/xenia/gpu/vulkan/vulkan_command_processor.h

@@ -145,6 +145,10 @@ class VulkanCommandProcessor final : public CommandProcessor {
 
   void TracePlaybackWroteMemory(uint32_t base_ptr, uint32_t length) override;
 
+  void InitializeShaderStorage(
+      const std::filesystem::path& cache_root, uint32_t title_id, bool blocking,
+      std::function<void()> completion_callback = nullptr) override;
+
   void RestoreEdramSnapshot(const void* snapshot) override;
 
   ui::vulkan::VulkanDevice* GetVulkanDevice() const {