ipn/local: add the suggested exit node to the ipn bus (#16748)

fixes tailscale/corp#26369

The suggested exit node is currently only calculated during a localAPI request.
For older UIs, this wasn't a bad choice - we could just fetch it on-demand when a menu
presented itself.  For newer incarnations however, this is an always-visible field
that needs to react to changes in the suggested exit node's value.

This change recalculates the suggested exit node ID on netmap updates and
broadcasts it on the IPN bus.   The localAPI version of this remains intact for the
time being.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

ipn/ipnlocal/bus.go

@@ -156,5 +156,6 @@ func isNotableNotify(n *ipn.Notify) bool {
 		n.Health != nil ||
 		len(n.IncomingFiles) > 0 ||
 		len(n.OutgoingFiles) > 0 ||
-		n.FilesWaiting != nil
+		n.FilesWaiting != nil ||
+		n.SuggestedExitNode != nil
 }