Consider allowing instances to indicate they accept VC formatting options

[dlongley]

We could add a feature to allow an issuer instance that is configured to issue both DI proofs and an envelope format to accept an issue option from the client to choose either DI proofs (yes, no) and / or one particular envelope format, for the resulting VC.

Note: Currently only one envelope format is supported per instance, maybe we could allow more than one in instances that support this prospective feature, but require just one to be chosen when a VC issued.