Support for Organization groups (new in 26.6)

[josep-llodra]

Keycloak 26.6 added groups (and subgroups) inside organizations.

https://www.keycloak.org/docs/latest/server_admin/index.html#proc-managing-groups_server_administration_guide

https://www.keycloak.org/docs-api/latest/rest-api/index.html#_get_adminrealmsrealmorganizationsorg_idgroups

I think the library does not support this yet, just filling the issue. I can help testing this in a real world application.

Thank you