Use webfinger for OIDC parameter discovery

[rhafer]

To solve many of the issues our users have with getting OpenCloud to work with different IDPs we
came up with a more generic way for how clients should discover their OIDC setttings (issuer, client_id and scopes). This is described here: https://github.com/opencloud-eu/opencloud/blob/main/docs/adr/0003-oidc-client-config-discovery.md

And implemented in: opencloud-eu/opencloud#2072

Now we need to implement this mechanism in our clients.