Switch back to use Product as the credentialSubject

This PR follows discussions with @revstifeev on !556 (closed) as well as discussions with Benjamin Young from digitalbazaar, regarding being able to keep the credentialSubject as a Product while also maintaining a base Product which can be re-used.

See the most recent comment summarising the discussion with Benjamin for more information.

I've separated the unrelated changes to SecureLink/Link into a separate branch so this branch only has the changes relevant to the DPP credentialSubject

Replaces: !556 (closed) Ref #540