idp: add the ipaidpuser objectclass when needed
    
    The ipaidpuser objectclass is required for the attribute ipaidpsub.
    When a user is created or modified with --idp-user-id, the operation
    must ensure that the objectclass is added if missing.
    
    Add a test for user creation and user modification with --idp-user-id.
    Fixes: https://pagure.io/freeipa/issue/9433
    
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>