Add helper for fetching userinfo (OIDC)

[jberger]

In OIDC the userinfo may be included in the authorization response/id token but it does not have to be so. For example Azure AD can provide some claim information in the authorization response/id token but to get the full set of information you need to call that userinfo_url fetched from the warmup.

Luckily the request is pretty easy: https://openid.net/specs/openid-connect-core-1_0.html#UserInfo

[jberger]

Ok, well after some debugging I was able to get what I needed from the id token after all. Still it might be nice to have easier access to the userinfo endpoint

[tomk3003]

@jberger could you please share what you did to extract the userinfo from the id token?

[jberger]

my $oidc_user = $c->oauth2->jwt_decode(deft => data => $provider_res->{id_token});

[jberger]

As OIDC is becoming even more commonplace, I would support more documentation and/or more helpers towards OIDC use here.

[tomk3003]

Thanks, I'll try that. And thanks for the info about 'response_type' in #70.
I had started to patch this myself, I because keycloak also insists on having it set.