I've succesfully implemented an OAuth2/OIDC provider using authlib. But now stumbled upon a issue where a clients is not able to actually use the id_token (in addition to the access token, it can only use one token at a time). This shouldn't be an issue, as there's always the userinfo endpoint that can be read using the access token (or so I tought). I looked into adding the userinfo endpoint, but cannot find a implementation in authlib.

Describe the solution you'd like

I'd like there to be a (reference) implementation of the userinfo endpoint as described in section 5.3 of the OpenID Connect Core 1.0 spec.