`kid` prop in JWTs

[bumblefudge]

Do we need normative or non-normative guidelines on:

• issuer DIDs with more than 1 key
• issuing VCs with a kid in the header when issuer DID has more than 1
• issuer SHOULD provide a kid even if there's only 1 for systems that might fumble the resolution "to default"
  etc?

cc @ra-phael for noticing in our testing VC!

[kimdhamilton]

Planning to address this in latest PR by always including kid in header, for clarity