Allow mutliple VCs in VP and validate VP without VC

[sake]

The patches contain two changes.

1. A VP may contain multiple VCs, so the VerifiablePresentation class is modified to retrieve a list of VCs. A possible use multiple VCs is that a self signed VC containing user asserted data might be presented together with a VC asserted by a third party.
2. VPs might contain no VC. The Validation class enforces the presence of a VC, however OID4VCI uses ldp proofs to claim key ownership. Such a VP contains no VC. In order to retain backwards compatibility, a boolean flag in has been introduced in the validate method.