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_test.go

@@ -209,6 +209,20 @@ func TestUndeltaPeers(t *testing.T) {
 				Key:  key.NodePublicFromRaw32(mem.B(append(make([]byte, 31), 'A'))),
 			}),
 		},
+		{
+			name: "change_key_signature",
+			prev: peers(n(1, "foo")),
+			mapRes: &tailcfg.MapResponse{
+				PeersChangedPatch: []*tailcfg.PeerChange{{
+					NodeID:       1,
+					KeySignature: []byte{3, 4},
+				}},
+			}, want: peers(&tailcfg.Node{
+				ID:           1,
+				Name:         "foo",
+				KeySignature: []byte{3, 4},
+			}),
+		},
 		{
 			name: "change_disco_key",
 			prev: peers(n(1, "foo")),