fix: "Product" interfaces shared fields fix

[kidunot89]

…re optimal schema shape

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What does this implement/fix? Explain your changes.

• Adds Product interface as parent interface to the following interfaces:
  • InventoriedProduct
  • ProductWithPricing
  • ProductWIthVariations
  • DownloadableProduct
  • ProductWithDimensions
  • ProductWithAttributes
  • ProductUnion

Does this close any currently open issues?

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