ipa user-add: add optional objectclass for radius-username
    
    The command "ipa user-add --radius-username" fails with
    ipa: ERROR: attribute "ipatokenRadiusUserName" not allowed
    because it does not add the objectclass ipatokenradiusproxyuser
    that is required by the attribute ipatokenradiususername.
    
    The issue happens with ipa user-add / stageuser-add / user-mod / stageuser-mod.
    
    The fix adds the objectclass when needed in the pre_common_callback method
    of baseuser_add and baseuser_mod (ensuring that user and stageuser commands
    are fixed).
    
    Fixes https://pagure.io/freeipa/issue/7569
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>