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.cpp

@@ -0,0 +1,47 @@
+// Copyright 2020 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include <memory>
+#include <httplib.h>
+#include "common/logging/log.h"
+#include "web_service/nus_download.h"
+
+namespace WebService::NUS {
+
+std::optional<std::vector<u8>> Download(const std::string& path) {
+    constexpr auto HOST = "http://nus.cdn.c.shop.nintendowifi.net";
+
+    std::unique_ptr<httplib::Client> client = std::make_unique<httplib::Client>(HOST);
+    if (client == nullptr) {
+        LOG_ERROR(WebService, "Invalid URL {}{}", HOST, path);
+        return {};
+    }
+
+    httplib::Request request{
+        .method = "GET",
+        .path = path,
+    };
+
+    client->set_follow_location(true);
+    const auto result = client->send(request);
+    if (!result) {
+        LOG_ERROR(WebService, "GET to {}{} returned null", HOST, path);
+        return {};
+    }
+
+    const auto& response = result.value();
+    if (response.status >= 400) {
+        LOG_ERROR(WebService, "GET to {}{} returned error status code: {}", HOST, path,
+                  response.status);
+        return {};
+    }
+    if (!response.headers.contains("content-type")) {
+        LOG_ERROR(WebService, "GET to {}{} returned no content", HOST, path);
+        return {};
+    }
+
+    return std::vector<u8>(response.body.begin(), response.body.end());
+}
+
+} // namespace WebService::NUS