control/controlclient,tailcfg: [capver 40] create KeySignature field in tailcfg.Node

We calve out a space to put the node-key signature (used on tailnets where network lock is enabled).

Signed-off-by: Tom DNetto <tom@tailscale.com>

control/controlclient/map.go

@@ -310,6 +310,9 @@ func undeltaPeers(mapRes *tailcfg.MapResponse, prev []*tailcfg.Node) {
 				if v := ec.Capabilities; v != nil {
 					n.Capabilities = *v
 				}
+				if v := ec.KeySignature; v != nil {
+					n.KeySignature = v
+				}
 			}
 		}
 	}