doc/designs: add External IdP support design documents
    
    External IdP objects represent OAuth 2.0 clients that can be used to
    perform OAuth 2.0 device authorization grant flow.
    
    Related: https://pagure.io/freeipa/issue/8805
    Related: https://pagure.io/freeipa/issue/8804
    Related: https://pagure.io/freeipa/issue/8803
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Francisco Trivino <ftrivino@redhat.com>
    Reviewed-By: Sumit Bose <sbose@redhat.com>