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"
]
```

The payload names only the connector-basic plugin and an OAuth2/OIDC provider group claim, with no files or tests. Start by locating the plugin’s login and claims-mapping entry points, then clarify how the `groups` value should map to Apache Answer roles; done means roles such as Administrators are loaded at login without manual assignment.

Written by the indexing model from the issue text.