Implement ADR 37

[JohnVonNeumann]

https://github.com/wbcsd/data-exchange-protocol/blob/main/decisions/log/0037-replace-productcategorycpc-with-productclassifications.md

Included URN namespace syntax for product classifications (ADR37)

Added optional property productClassifications (ADR37)

Advisement that property productCategoryCpc will be deprecated in version 3 (ADR37)

This is related to #70