Add sku and mpn to product schema

[dpotocic]

Fixes two search console warnings:
Missing field "sku"
No global identifier provided (e.g., gtin, mpn, isbn)

[JoppeDC]

Hi @dpotocic . Thanks for your PR. Adding the SKU, i can approve. But the MPN is not the same as the SKU, and Sylius has no field provided for this. We can either add a field for this, or remove it from the json.
Please note that these warnings should have no impact on the result of the snippet.