feat(airvpn): add AirVPN support

[qdm12]

To try it out...

OpenVPN

1. Create a directory /yourpath on your host machine

2. Place your user.crt file in /yourpath/client.crt

3. Place your user.key file in /yourpath/user.key

4. Get the image either by pulling docker pull qmcgaw/gluetun:pr-1145 or building it with docker build -t qmcgaw/gluetun:pr-1145 https://github.com/qdm12/gluetun.git#airvpn

5. Run it with

   docker run -it --rm --cap-add=NET_ADMIN -e VPN_SERVICE_PROVIDER=airvpn \
   -v /yourpath:/gluetun qmcgaw/gluetun:pr-1145

Wireguard

docker run -it --rm --cap-add=NET_ADMIN -e VPN_SERVICE_PROVIDER=airvpn \
-e VPN_TYPE=wireguard \
-e WIREGUARD_PRIVATE_KEY=yCbHtKAgJASnJs2S/aZsjag9Fy04csmR4qDUDkJgX2c= \
-e WIREGUARD_PRESHARED_KEY=yCbHtKAgJASnJs2S/aZsjag9Fy04csmR4qDUDkJgX2c= \
-e WIREGUARD_ADDRESSES=10.99.99.99/32,ff:ff:ff...:ff/128 \
qmcgaw/gluetun:pr-1145