PIA nextgen portforward (#242)

* Split provider/pia.go in piav3.go and piav4.go
* Change port forwarding signature
* Enable port forwarding parameter for PIA v4
* Fix VPN gateway IP obtention
* Setup HTTP client for TLS with custom cert
* Error message for regions not supporting pf

internal/models/selection.go

@@ -90,6 +90,7 @@ func (p *ProviderSettings) String() string {
 		settingsList = append(settingsList,
 			"Region: "+p.ServerSelection.Region,
 			"Encryption preset: "+p.ExtraConfigOptions.EncryptionPreset,
+			"Port forwarding: "+p.PortForwarding.String(),
 		)
 	case "mullvad":
 		settingsList = append(settingsList,