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

internal/vpn/tunnelup.go

@@ -12,6 +12,8 @@ type tunnelUpData struct {
 	vpnIntf        string
 	serverName     string // used for PIA
 	canPortForward bool   // used for PIA
+	username       string // used for PIA
+	password       string // used for PIA
 	portForwarder  PortForwarder
 }