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

While web already has a way to configures these settings, all our clients should use the same mechanism for these settings.

As soon as web has implemented this we should also remove/deprecate the respective settings in the web service. I think that would be WEB_OIDC_METADATA_URL, WEB_OIDC_AUTHORITY, WEB_OIDC_CLIENT_ID and WEB_OIDC_SCOPE

[rhafer]

cc @kulmann @db-ot