add domain/challenge to vp signing; only return jwt from signing

[domleboss97]

• Updates the signCredential and signPresentation to only return JWTs, rather than requiring a resolver to immediately parse the credential and return the parsed object. This more closely aligns with the intention of using jwt credentials. App logic can always call parseJwtCredential to get the parsed credential.
• Update signPresentation to allow passing through a challenge and a domain.
• Export verifyPresentation.