tailcfg: add a field to advertise support for IPv6 tailscale config.

Signed-off-by: David Anderson <danderson@tailscale.com>

control/controlclient/direct.go

@@ -45,6 +45,10 @@ import (
 	"tailscale.com/version"
 )
 
+var (
+	enableV6Overlay, _ = strconv.ParseBool(os.Getenv("TS_DEBUG_ENABLE_IPV6_OVERLAY"))
+)
+
 type Persist struct {
 	_ structs.Incomparable
 
@@ -527,6 +531,7 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 	request := tailcfg.MapRequest{
 		Version:            4,
 		IncludeIPv6:        true,
+		IncludeIPv6Overlay: enableV6Overlay,
 		DeltaPeers:         true,
 		KeepAlive:          c.keepAlive,
 		NodeKey:            tailcfg.NodeKey(persist.PrivateNodeKey.Public()),

tailcfg/tailcfg.go

@@ -468,6 +468,7 @@ type MapRequest struct {
 	DiscoKey           DiscoKey
 	Endpoints          []string // caller's endpoints (IPv4 or IPv6)
 	IncludeIPv6        bool     // include IPv6 endpoints in returned Node Endpoints
+	IncludeIPv6Overlay bool     // include IPv6 Addresses and AllowedIPs in returned Nodes.
 	DeltaPeers         bool     // whether the 2nd+ network map in response should be deltas, using PeersChanged, PeersRemoved
 	Stream             bool     // if true, multiple MapResponse objects are returned
 	Hostinfo           *Hostinfo