Expose discovered config

I would like to access the `userinfo_endpoint` provided by the discovery endpoint and already required during the `authorize` , and later on use it to get this user info.
As of today I need to make the same call to the discover to grab the `userinfo_endpoint` and use it to fetch the profile information of the user.
I would avoid a rountrip for the already required config if it is possible to just access it.

See: https://github.com/FormidableLabs/react-native-app-auth/blob/bacd0377b46c0f0196a8b5cf86553484feacb023/ios/RNAppAuth.m#L112