Support predicates in proof-requests

[esune]

Some use cases seem to be open to using predicates in proof requests, for scenarios such as proving someone holds a credential from a specific issuer to authenticate without sharing additional information, or validating the credential being presented meets specific restrictions for field values (i.e.: expiration dates).

• What do we need to implement in vc-authn to support predicates?
• Define a way to template predicates with values - such as current date - that need to be calculated at presentation compilation time.
• How well will the OIDC flow work in the case of ZKPs, where no data is shared at all?

[loneil]

#626