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/provider/utils/portforward.go

@@ -19,6 +19,10 @@ type PortForwardObjects struct {
 	ServerName string
 	// CanPortForward is used by Private Internet Access for port forwarding.
 	CanPortForward bool
+	// Username is used by Private Internet Access for port forwarding.
+	Username string
+	// Password is used by Private Internet Access for port forwarding.
+	Password string
 }
 
 type Routing interface {