Add support for advanced BBS features like pseudonyms

[msporny]

Protocols, such as VC API, that support unlinkable disclosure schemes (such as BBS or CL Signatures) with features such as pseudonyms and binding to a secret that a holder possesses, need to provide mechanisms to achieve those features. The VC API should add fields (like prover ids and verifier ids and commitments) to the protocol (perhaps via vp-request-spec) and examples of how to achieve these advanced features.

/cc @Wind4Greg

[msporny]

These features need to be incubated in the VC Playground and a wallet or two before we add the text to the specification.