Replaces DIY natpmpc sidecar with gluetun's built-in NAT-PMP handling for the protonvpn
provider. The natpmpc UDP response was being dropped by gluetun's firewall since conntrack
doesn't track stateless UDP from the gateway. With VPN_PORT_FORWARDING=on, gluetun handles
the NAT-PMP exchange internally and exposes the port at :8000/v1/openvpn/portforwarded.
Helper sidecar now just polls that endpoint.
Will Anderson
e1996d4396