Feature request: Resolve hostname of VPN server

[qdm12]

What's the feature? 🧐

Resolve the hostname of the VPN server at start, this is especially useful for custom openvpn configuration file (#223).

• Env variable to enable this, as this add a security hole in the firewall
• Allow DNS through firewall out of tunnel at start only, then block it
• Only allow the DNS Resolution for that particular server hostname to avoid leaking DNS queries. To do so we should have a custom DNS forwarding implementation, so that's blocked by #137