Define the AnonCreds v2 Presentation Request format

[swcurran]

AnonCreds v2 defines the presentation statements as the building blocks of what must be presented in an AnonCreds v2 presentation. A structure above the presentation statements are needed to enable the multi-source credential presentation requests that are supported in AnonCreds v1. The format need not be identical to AnonCreds v1, and even if that format were the basis, additional expressions would be required to support the expanded ZKP capabilities. An alternate option is the DIF Presentation Exchange format.

Task is to decide on a format and define how to transform the selected format into the presentation statements needed in producing an AnonCreds v2 proof.