20220802 fix nextcloud (#1750)

* Avoid Nextcloud "Current user is not logged in" (Statuscode 997)

The error message results from oauth2-proxy trying to pass the
access token via URL. Instead it needs to be sent via header,
thus the Nextcloud provider requires a fix similar to what #1502
did before for the keycloak provider.

* Implement EnrichSession() for Nextcloud provider

Parse nested JSON to transform relevant information (groups, id,
email) from the OAuth2 userinfo endpoint into session.

* Update CHANGELOG.md (add link to PR #1750)