Feature request: Add ability to set "exp" and "nbf" for JWT formatted verifiable presentations

[vdods]

Verifiable Credential (ssi::vc::Credential) has expiration_date and not_before fields which map onto the exp and nbf claims of a JWT, but Verifiable Presentation (ssi::vc::Presentation) is missing those, so it's not possible to properly time-limit a VP.

Would adding this capability be as simple as copying those fields from ssi::vc::Credential into ssi::vc::Presentation and hooking up appropriate plumbing? Or is there an active reason it's missing? If it's the former, I'd be happy to take a crack at it.