Implement issuing a verifiable presentation

[sgreenbury]

Include functionality for transforming from a VC to a VP. This process relates to the W3C VC API. See work in didkit for reference.

[sgreenbury]

• Explore Presentation and didkit implementation
• Consider requirements with respect to FFI (FFI implementation for mobile #62, FFI implementation for desktop #63 )

[edchapman88]

Updates:

• Changes to Issuer.sign() and Holder.sign_presentation() to include Option<LinkedDataProofOptions> in the api.
  • For Issuer.sign(): if None, the default is used (which sets ProofPurpose::AssertionMethod).
  • For Holder.sign_presentation(): if None, ProofPurpose::Authentication is set along with the other fields from Default::default().
• Change to sign_presentation() to ensure the holder field is set on presentations before they are signed.
• Presentation.generate_proof() attempts to resolve a DID provided in the holder field of the presentation.
  • This has implications for supporting the partial verification of presentations in the cases where the holder DID is either non-resolvable or non-verifiable.
  • It is not currently possible to sign presentations with non-resolvable DIDs