Obtain PIA v4 server information from API (#257)

- Obtain CN for port forwarding https verification
- Obtain for each server if they support port forwarding
- Obtain for each server their IP address for openvpn UDP and openvpn TCP (one for each)
- Updater program updated to use API
- Hardcoded values updated for PIA v3 and v4 servers
- Clearer separation between pia v3 and v4
- Fixes #250

internal/provider/piav4.go

@@ -161,6 +161,71 @@ func (p *piaV4) PortForward(ctx context.Context, client *http.Client,
 	}
 }
 
+func filterPIAServers(servers []models.PIAServer, region string) (filtered []models.PIAServer) {
+	if len(region) == 0 {
+		return servers
+	}
+	for _, server := range servers {
+		if strings.EqualFold(server.Region, region) {
+			return []models.PIAServer{server}
+		}
+	}
+	return nil
+}
+
+func getPIAOpenVPNConnections(allServers []models.PIAServer, selection models.ServerSelection) (connections []models.OpenVPNConnection, err error) {
+	servers := filterPIAServers(allServers, selection.Region)
+	if len(servers) == 0 {
+		return nil, fmt.Errorf("no server found for region %q", selection.Region)
+	}
+
+	var port uint16
+	switch selection.Protocol {
+	case constants.TCP:
+		switch selection.EncryptionPreset {
+		case constants.PIAEncryptionPresetNormal:
+			port = 502
+		case constants.PIAEncryptionPresetStrong:
+			port = 501
+		}
+	case constants.UDP:
+		switch selection.EncryptionPreset {
+		case constants.PIAEncryptionPresetNormal:
+			port = 1198
+		case constants.PIAEncryptionPresetStrong:
+			port = 1197
+		}
+	}
+	if port == 0 {
+		return nil, fmt.Errorf("combination of protocol %q and encryption %q does not yield any port number", selection.Protocol, selection.EncryptionPreset)
+	}
+	for _, server := range servers {
+		IPs := server.OpenvpnUDP.IPs
+		if selection.Protocol == constants.TCP {
+			IPs = server.OpenvpnTCP.IPs
+		}
+		for _, IP := range IPs {
+			if selection.TargetIP != nil {
+				if selection.TargetIP.Equal(IP) {
+					return []models.OpenVPNConnection{{IP: IP, Port: port, Protocol: selection.Protocol}}, nil
+				}
+			} else {
+				connections = append(connections, models.OpenVPNConnection{IP: IP, Port: port, Protocol: selection.Protocol})
+			}
+		}
+	}
+
+	if selection.TargetIP != nil {
+		return nil, fmt.Errorf("target IP %s not found in IP addresses", selection.TargetIP)
+	}
+
+	if len(connections) > 64 {
+		connections = connections[:64]
+	}
+
+	return connections, nil
+}
+
 func newPIAv4HTTPClient() (client *http.Client, err error) {
 	certificateBytes, err := base64.StdEncoding.DecodeString(constants.PIACertificateStrong)
 	if err != nil {