ipasam: add lookup of an account by SID
    
    Samba may ask for an account based on a SID value. Implement a callback
    to return a result of such lookup since we should have SID for every
    domain account that is supposed to be usable through SMB protocol.
    
    Fixes: https://pagure.io/freeipa/issue/3999
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>