Implement SHOULD behaviour for 4.21 ProductId

[JohnVonNeumann]

TODO: Implement the additional functionality to support:
4.21.1. Custom Product Ids (Product Codes)
4.21.2. ProductId based on CAS Registry Numbers
4.21.3. ProductId based on IUPAC InChi Code

https://wbcsd.github.io/tr/2023/data-exchange-protocol-20230221/#dt-productid

[JohnVonNeumann]

Implemented simply to start with, ignored the SHOULD behaviours as the MUST behaviour was satisfied by ensuring URN.