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/run.go

@@ -50,6 +50,8 @@ func (l *Loop) Run(ctx context.Context, done chan<- struct{}) {
 			canPortForward: canPortForward,
 			portForwarder:  portForwarder,
 			vpnIntf:        vpnInterface,
+			username:       settings.Provider.PortForwarding.Username,
+			password:       settings.Provider.PortForwarding.Password,
 		}
 
 		openvpnCtx, openvpnCancel := context.WithCancel(context.Background())