control,health,ipn: move IP forwarding check to health tracker (#19007)

Currently IP forwarding health check is done on sending MapRequests.

Move ip forwarding to the health service to gain the benefits
of the health tracker and perodic monitoring out of band from
the MapRequest path. ipnlocal now provides a closure to
the health service to provide the check if forwarding is broken.

Removed `skipIPForwardingCheck` from controlclient/direct.go,
it wasn't being used as the comments describe it, that check
has moved to ipnlocal for the closure to the health tracker.

Updates #18976

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

control/controlclient/client.go

@@ -87,6 +87,9 @@ type Client interface {
 	// future map requests. This should be called after rotating the discovery key.
 	// Note: the auto client uploads the new key to control immediately.
 	SetDiscoPublicKey(key.DiscoPublic)
+	// SetIPForwardingBroken updates the IP forwarding broken state
+	// and sends a control update if the value changed.
+	SetIPForwardingBroken(bool)
 	// ClientID returns the ClientID of a client. This ID is meant to
 	// distinguish one client from another.
 	ClientID() int64