idviews: add extended validator for users from trusted domains
    
    Register extended validator for users from trusted domains to be called
    through add_external_pre_callback() in sudorules and other plugins.
    
    The callbacks allow to validate user names as following:
    
     - if user name passes basic user name validator it is accepted, otherwise
     - if user name can be resolved to any user in IPA or in a trusted
       domain, it is accepted
     - otherwise the name is rejected
    
    Fixes: https://pagure.io/freeipa/issue/3226
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>