cmd/natc: add optional consensus backend

Enable nat connector to be run on a cluster of machines for high
availability.

Updates #14667

Signed-off-by: Fran Bull <fran@tailscale.com>

cmd/natc/natc_test.go

@@ -270,7 +270,7 @@ func TestDNSResponse(t *testing.T) {
 		ignoreDsts: &bart.Table[bool]{},
 		routes:     routes,
 		v6ULA:      v6ULA,
-		ipPool:     &ippool.IPPool{IPSet: addrPool},
+		ipPool:     &ippool.SingleMachineIPPool{IPSet: addrPool},
 		dnsAddr:    dnsAddr,
 	}
 	c.ignoreDsts.Insert(netip.MustParsePrefix("8.8.4.4/32"), true)