Allow resolve() to return abstract DID document (option 1)

[peacekeeper]

Building on #296 and #297, this is one of two proposals how resolve() can return a DID document in an abstract form.

• Proposal 1 Allow resolve() to return abstract DID document (option 1) #322 (this one): resolve() always returns a DID document in an abstract form, and dereference() always returns a DID document or other content as a byte stream of a concrete representation.
• Proposal 2 Allow resolve() to return abstract DID document (option 2) #323: resolve() can return either a DID document in an abstract form or as a byte stream of a concrete representation, depending on whether the accept input metadata property is present.

[msporny]

Ok with this one, but prefer #323.

[dlongley]

I prefer #323 -- I'm concerned here with introducing more things that others might find objectionable.

[peacekeeper]

Replaced by #331 per discussion on the 2020-06-18 DID WG Topical Call on DID Resolution contracts.