Maint: make VPN connection not specific to OpenVPN

- Add VPN field to ServerSelection struct
- Set VPN type to server selection at start using VPN_TYPE
- Change OpenVPNConnection to Connection with Type field
- Rename Provider GetOpenVPNConnection to GetConnection
- Rename GetTargetIPOpenVPNConnection to GetTargetIPConnection
- Rename PickRandomOpenVPNConnection to PickRandomConnection
- Add 'OpenVPN' prefix to OpenVPN specific methods on connection

internal/provider/utils/pick.go

@@ -6,7 +6,7 @@ import (
 	"github.com/qdm12/gluetun/internal/models"
 )
 
-func PickRandomOpenVPNConnection(connections []models.OpenVPNConnection,
-	source rand.Source) models.OpenVPNConnection {
+func PickRandomConnection(connections []models.Connection,
+	source rand.Source) models.Connection {
 	return connections[rand.New(source).Intn(len(connections))] //nolint:gosec
 }