Updates DPP to use new Jargon strategy to keep Product as credentialSubject

Tested DPP for valid jsonld and schema with https://github.com/absoludity/jargon-untp-tester

See #540 for more details.

Ref #540

The actual Jargon changes to generate this was:

• Add [jargon.aggregation] to ProductPassport.product , so that rather than being a separate product property, all the properties of product are added for ProductPassport, and
• sets ProductPassport's own @id to untp:Product so that it masquerades as the Product.

This allows us to have a separate object defined in Jargon, ProductPassport which has all the fields of Product as well as some extra ones (claims etc.) (benefits of a schema and sample data), while the expanded JSON-LD just sees a Product with extra relations/predicates, as is right for JSON-LD.

See #540 for more details.