Refactor signing verifiable credentials

[mistermoe]

signing a verifiable credential is a bit rough around the edges at the moment - example here. Specifically,

• alg is hardcoded to EdDSA. This makes passing in a signer function as an argument unnecessary because there's only one possible DSA that can be used: Ed25519.
• subjectDid is required when calling sign, but also required when calling create. I think we can remove it as a required argument for sign

@frankhinek how do you imagine sign's method signature will change given the changes proposed in #271?

[frankhinek]

Thanks for the feedback @mistermoe ! Great to have someone building to reveal how the SDK can be improved.

• Agreed on the header and JWT payload details ought to be customizable.
• Yes, once we can agree on our approach to a consistent set of Key Generators, Signers, etc. that will require making changes across the DID and VC public API.

[mistermoe]

First pass of this is done. sign was refactored to take just a DID. will open other more focused issues as they crop up