stageuser: add ipauserauthtypeclass when required
    
    The command
    ipa stageuser-add --user-auth-type=xxx
    is currently failing because the objectclass ipauserauthtypeclass
    is missing from the created entry.
    
    There is code adding the missing objectclass in the
    pre_common_callback method of user_add, and this code should
    be common to user_add and stageuser_add. In order to avoid code
    duplication, it makes more sense to move the existing code to
    pre_common_callback of baseuser_add, that is called by both
    classes.
    
    Fixes: https://pagure.io/freeipa/issue/8909
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>