Add controlled list of "external identifers"

[DsposalTom]

Should we think about adding a controlled list of the externals identifiers.

I am thinking that using a subset of schema.org's product schema? Obvs we don't need it all but it might be worth a conversation.

[DsposalTom]

I think this would be a minor change and not a patch - moving to Version 3.0 milestone.

[DsposalTom]

These are some examples I found in Schema.org product.

• brand
• gtin
• gtin12
• gtin13
• gtin14
• gtin8
• hasGS1DigitalLink
• productID
• sku

I think we should also include

• primaryKey