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/openvpn/custom/custom.go

@@ -14,7 +14,7 @@ var (
 )
 
 func BuildConfig(settings configuration.OpenVPN) (
-	lines []string, connection models.OpenVPNConnection, err error) {
+	lines []string, connection models.Connection, err error) {
 	lines, err = readCustomConfigLines(settings.Config)
 	if err != nil {
 		return nil, connection, fmt.Errorf("%w: %s", ErrReadCustomConfig, err)