support flattening of nested groups like keycloak uses

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Feature request

Which Nextcloud Version are you currently using: 29.0.6

Is your feature request related to a problem? Please describe.
In Keycloak it is possible to have nested (hierarchical) groups like "/parent/child". So if a user is member of child group it is implicitly also member of parent group.
Nextcloud doesn't support nested groups.
Unfortunately at the time being keycloak can only send the whole group path "/parent/child" or only the group the user is explicitly member of, so "child'.
In a lot of use cases, having an option which supports flattening the groups would be enough. Like
"/parent/child" becomes the two groups:

• parent
• child

Would be nice if that would be an selectable option for the user_oidc app.

Start by tracing how the user_oidc app reads group claims from Keycloak and where its user-selectable options are configured. Done means an option can flatten a path such as /parent/child into both parent and child groups, while preserving the existing group behavior.