Enhancement: Authenticated WebFinger

Authenticated WebFinger was introduced into the authentication flow.
Now, WebFinger is used to retrieve the OpenID Connect issuer and the available ownCloud instances.
For the moment, multiple oC instances are not supported, only the first available instance is used.

https://github.com/owncloud/android/issues/3943
https://github.com/owncloud/android/pull/3945
https://doc.owncloud.com/ocis/next/deployment/services/s-list/webfinger.html
