Add did-jwt-vc tests

[kimdhamilton]

Added did-jwt-vc tests, focusing only on signing and ed25519 to start with. Skeleton of the rest is there.

Specifically, signing tests for the following pass:

• credential-1--key-0-ed25519.vc-jwt.json
• credential-2--key-0-ed25519.vc-jwt.json
• credential-3--key-0-ed25519.vc-jwt.json
• presentation-0--key-0-ed25519.vp-jwt.json
• presentation-1--key-0-ed25519.vp-jwt.json

Note that there is a lot of duplication with other js implementations (bin and cli). Addressing that seemed out of scope.

As for implementing the rest -- this test suite is a little onerous so I had to timebox the work. But I included the framework so that it give subsequent people a good head start:

• Support for additional signers should be easy for someone with the JWK mappings in active recall.
• Verification should be straightforward if someone can add a did:example resolver

[bumblefudge]

Addresses #39

[clehner]

Nice!
I see the following results, showing some successful Ed25519 VCs and VPs:

[OR13]

@clehner @peacekeeper I wonder if we can figure out how to consistently handle the "no support" side of this... so that it does not look like an implementation is failing when they don't support the key type or algorithm.