[yang] bgp address family l2vpn advertise-all-vni

Cannot configured unified bgp for vxlan evpn without specifying
advertise-all-vpn.  The setting appears to have been introduced
as part of PR sonic-net#5142

Signed-off-by: Brad House (@bradh352)

src/sonic-yang-models/yang-models/sonic-bgp-global.yang

@@ -445,6 +445,11 @@ module sonic-bgp-global {
                     }
                     description "Maximum duration to suppress a stable route";
                 }
+
+                leaf advertise-all-vni {
+                    type boolean;
+                    description "L2VPN advertise all VNIs";
+                }
             }
         }