Browser+LibWeb+WebContent: Allow Browser to load local files

To achieve this goal:
 - The Browser unveils "/tmp/portal/filesystemaccess"
 - Pass the page through LoadRequest => ResourceLoader
 - ResourceLoader requests a file to the FileSystemAccessServer via IPC
 - OutOfProcessWebView handles it and sends a file descriptor back to
 the Page.

Userland/Libraries/LibWeb/Loader/FileRequest.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2022, Lucas Chollet <lucas.chollet@free.fr>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+#include <AK/Function.h>
+#include <AK/RefCounted.h>
+#include <AK/String.h>
+
+namespace Web {
+
+class FileRequest : public RefCounted<FileRequest> {
+public:
+    explicit FileRequest(String path);
+
+    String path() const;
+
+    Function<void(ErrorOr<i32>)> on_file_request_finish;
+
+private:
+    String m_path {};
+};
+
+}