Consider including `recipientPublicJwk` (or similarly named) property in VPR for encrypted responses

[dlongley]

This is another issue related to helping to support DC-API or other requests to add an additional encryption layer. This property could be provided on a VPR, to request that any VPs given in response be encrypted. Each encrypted VP could then be expressed as an EnvelopedVerifiablePresentation with an appropriate media type (on its data URL), that would then decrypt to a VerifiablePresentation (or another EnvelopedVerifiablePresentation with some other media type).