feat: integrate login via OIDC

[spaceone]

Disabling the OIDC authentication can be done via:
ucr set appcenter/apps/nextcloud/disable-oidc-login=true

Setting the OIDC login identifier (displayed in the login dialog) can be achieved via:
ucr set appcenter/apps/nextcloud/oidc-identifier=UCS

Disabling the SAML authentication can be done via:
ucr set appcenter/apps/nextcloud/disable-saml-login=true

Changing these settings requires a new run of the joinscript.

Nextcloud by default doesn't permit accessing local networks, which
makes testing harder. It just says: Could not reach the OpenID Connect provider..

For testing purposes this can just be disabled via adding 'allow_local_remote_servers' => true, to /var/www/html/config/config.php.

Fixes: #204

[blizzz]

One thing I noticed, OIDC does not seem to play along with LDAP, and administrators are not promited ot NC admins either.

And on logout we end up with having the keycloak screen with an "Invalid redirect uri" error.

I think I'd revert for now and get the release out with 30 and saml only first.

[spaceone]

OK. Let's continue the discussion at #212.

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