Support customizable callback paths or multiple callback paths.

[CatGG-101]

Both /oauth/callback and / are common OAuth callback endpoints that should be supported to prevent existing OAuth servers from encountering redirect_uri_mismatch issues.

For example, the redirect_uris list should be able to include both:
http://127.0.0.1:3334/oauth/callback
and
http://127.0.0.1:3334/

Alternatively, a configurable path parameter could be implemented with default value set to /oauth/callback, while allowing customization to / or other values.