all: declare & plumb IPv6 masquerade address for peer

This PR plumbs through awareness of an IPv6 SNAT/masquerade address from the wire protocol
through to the low-level (tstun / wgengine). This PR is the first in two PRs for implementing
IPv6 NAT support to/from peers.

A subsequent PR will implement the data-plane changes to implement IPv6 NAT - this is just plumbing.

Signed-off-by: Tom DNetto <tom@tailscale.com>
Updates ENG-991

tailcfg/tailcfg_test.go

@@ -350,7 +350,7 @@ func TestNodeEqual(t *testing.T) {
 		"UnsignedPeerAPIOnly",
 		"ComputedName", "computedHostIfDifferent", "ComputedNameWithHost",
 		"DataPlaneAuditLogID", "Expired", "SelfNodeV4MasqAddrForThisPeer",
-		"IsWireGuardOnly", "ExitNodeDNSResolvers",
+		"SelfNodeV6MasqAddrForThisPeer", "IsWireGuardOnly", "ExitNodeDNSResolvers",
 	}
 	if have := fieldsOf(reflect.TypeOf(Node{})); !reflect.DeepEqual(have, nodeHandles) {
 		t.Errorf("Node.Equal check might be out of sync\nfields: %q\nhandled: %q\n",
@@ -545,6 +545,16 @@ func TestNodeEqual(t *testing.T) {
 			&Node{SelfNodeV4MasqAddrForThisPeer: ptr.To(netip.MustParseAddr("100.64.0.1"))},
 			true,
 		},
+		{
+			&Node{},
+			&Node{SelfNodeV6MasqAddrForThisPeer: ptr.To(netip.MustParseAddr("2001::3456"))},
+			false,
+		},
+		{
+			&Node{SelfNodeV6MasqAddrForThisPeer: ptr.To(netip.MustParseAddr("2001::3456"))},
+			&Node{SelfNodeV6MasqAddrForThisPeer: ptr.To(netip.MustParseAddr("2001::3456"))},
+			true,
+		},
 		{
 			&Node{
 				CapMap: NodeCapMap{