assign users to groups from OIDC claims

[jeriox]

closes #1105

[coveralls]

coverage: 88.124% (-0.003%) from 88.127% when pulling 03560be on oidc-groups into 55184b0 on main.

[lukasrad02]

From my, not Python- or Django-experienced perspective, only considering the overall OIDC stuff, this PR looks good.

Allowing dots for nested access only for the groups claim but not the other ones might seem like a random choice (especially for people not knwoing/using Keycloak's realm_access.roles default claim name). For consistency reasons, you might consider to add support for nested claims to all other claim identifiers or drop it from the groups.
Edit: Sorry, I did not notice that the other claims are hardcoded and use the OIDC default names, so the group claim is the only configurable one. But since there is no default for groups, your solution is fine.