Merge pull request #531 from Ecwid/ECWID-169265-upc

ECWID-169265 X+E: Stream additional product identifiers. Add additional product codes to the Google Shopping feed

Author: mamirovlspeed

src/main/kotlin/com/ecwid/apiclient/v3/dto/producttype/enums/AttributeType.kt

@@ -16,5 +16,6 @@ enum class AttributeType {
 	EXTERNAL_ISBN,
 	EXTERNAL_ITF,
 	EXTERNAL_JAN,
-	EXTERNAL_CUSTOM
+	EXTERNAL_CUSTOM,
+	EXTERNAL_UPC
 }