feat(pia): port forwarding options VPN_PORT_FORWARDING_USERNAME and VPN_PORT_FORWARDING_PASSWORD

- Retro-compatible with `OPENVPN_USER` + `OPENVPN_PASSWORD`
- No more reading for the OpenVPN auth file
- Allow to use PIA port forwarding with Wireguard

Dockerfile

@@ -120,6 +120,8 @@ ENV VPN_SERVICE_PROVIDER=pia \
     VPN_PORT_FORWARDING_LISTENING_PORT=0 \
     VPN_PORT_FORWARDING_PROVIDER= \
     VPN_PORT_FORWARDING_STATUS_FILE="/tmp/gluetun/forwarded_port" \
+    VPN_PORT_FORWARDING_USERNAME= \
+    VPN_PORT_FORWARDING_PASSWORD= \
     # # Cyberghost only:
     OPENVPN_CERT= \
     OPENVPN_KEY= \