Support for userinfo endpoint

[haakonst]

I needed this when working with Vipps Login as OIDC provider since it doesn't return any of the information requested in the scope as claims, but requires that it's obtained with a request to the userinfo endpoint.

[bcardarella]

This will need a test

[haakonst]

Added a test

[arjan]

Needing this too :-)

[bcardarella]

We're not actively using the project. I'm happy to add someone as collaborator and provide hex access to keep moving this forward.