Implement RomFS cache and async reads. (#7089)

* Implement RomFS cache and async reads.

* Suggestions and fix compilation.

* Apply suggestions

src/core/file_sys/file_backend.h

@@ -86,6 +86,20 @@ public:
      */
     virtual void Flush() const = 0;
 
+    /**
+     * Whether the backend supports cached reads.
+     */
+    virtual bool AllowsCachedReads() const {
+        return false;
+    }
+
+    /**
+     * Whether the cache is ready for a specified offset and length.
+     */
+    virtual bool CacheReady(std::size_t file_offset, std::size_t length) {
+        return false;
+    }
+
 protected:
     std::unique_ptr<DelayGenerator> delay_generator;