Load roles from OAuth2

I'm managing all users with SSO/OIDC/OAuth2 provider and there I also have groups/roles etc.

It looks like currently using `connector-basic` plugin it's not possible to load roles like Admin/Moderator from OAuth2 provider. This means after each user logins I would have to assign them that manually.

In my provider it has them like this:
```
"groups": [
        "Administrators"
]
```
