Add option to configure to download system files from Nintendo Update Service (#6269)

Co-authored-by: B3n30 <benediktthomas@gmail.com>

src/web_service/nus_download.h

@@ -0,0 +1,15 @@
+// Copyright 2020 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <optional>
+#include <vector>
+#include "common/common_types.h"
+
+namespace WebService::NUS {
+
+std::optional<std::vector<u8>> Download(const std::string& path);
+
+}