Add configurable URL hooks

Signed-off-by: Sean Du <do4suki@gmail.com>

src/config.hpp

@@ -0,0 +1,11 @@
+#pragma once
+
+namespace config {
+    struct OverrideUrls {
+        std::string api_url;
+        std::string asset_url;
+    };
+
+    void load();
+    const OverrideUrls& get();
+}