Now this was a lot of work for some reason.
I statically included the OpenIDConnectClient from Jumbojett
(https://github.com/jumbojett/OpenID-Connect-PHP/tree/v0.9.2)
and the phpseclib version 2 and it somehow works.
Currently though, if both internal and Oauth logins are enabled, it will
create a conflict if someone from the OAuth service tries to log in who
has got the same name as the internal user, so I recommend only using
one Login system

I hope I did not somehow violate any License