network: add auto-detection of optimal MTU for WireGuard interface, --wg-mtu flag, set max_mtu for Corrosion to 1232

internal/machine/network/mtu_darwin.go

@@ -0,0 +1,10 @@
+//go:build darwin
+
+package network
+
+import "errors"
+
+// detectEgressMTU is a stub for Darwin. The machine daemon that performs detection only runs on Linux.
+func detectEgressMTU() (int, error) {
+	return 0, errors.New("not implemented on darwin")
+}