wgengine/router,ipn/ipnlocal: add MTU field to router config

The MacOS client can't set the MTU when creating the tun due to lack
of permissions, so add it to the router config and have MacOS set it
in the callback using a method that it does have permissions for.

Updates #8219

Signed-off-by: Val <valerie@tailscale.com>

wgengine/router/router.go

@@ -67,6 +67,11 @@ type Config struct {
 	// routing rules apply.
 	LocalRoutes []netip.Prefix
 
+	// NewMTU is currently only used by the MacOS network extension
+	// app to set the MTU of the tun in the router configuration
+	// callback. If zero, the MTU is unchanged.
+	NewMTU int
+
 	// Linux-only things below, ignored on other platforms.
 	SubnetRoutes     []netip.Prefix         // subnets being advertised to other Tailscale nodes
 	SNATSubnetRoutes bool                   // SNAT traffic to local subnets