Add support of product combination

[DmitriyKulagin]

At the moment it is not possible to generate information about product combinations. This is partially solved by having custom fields such as "Age group", "Color" and "Size". This way you can only create unique products to tag them with a special identifier like SKU and GTIN.

https://support.google.com/merchants/answer/6324461?hl=en&ref_topic=6324338#zippy=%2Cproducts-with-several-gtins%2Cproducts-available-in-variant-colors-or-sizes