Finish GTIN mapping

[dubnemo]

Finish GTIN mapping; not able to map to Product, but must be added as a ContextItem

                    // map to contentItems
                    //
                    // gtin , add this to unique identifer on product?
                    //
                    var gtin = shippedItemInstance.Item.Gtinid;
                    _logger.LogInformation("GTIN = " + gtin);
                    //
                    // Where is gtin used?
                    //