* feat(keycloak_client): add valid_post_logout_redirect_uris and backchannel_logout_url
Add two new convenience parameters that map to client attributes:
- valid_post_logout_redirect_uris: sets post.logout.redirect.uris
attribute (list items joined with ##)
- backchannel_logout_url: sets backchannel.logout.url attribute
These fields are not top-level in the Keycloak REST API but are stored
as client attributes. The new parameters provide a user-friendly
interface without requiring users to know the internal attribute names
and ##-separator format.
Fixes#6812, fixes#4892
* consolidate changelog and add PR link per review feedback
