[OID4VP] VP token validation logic in the Cloud Agent

[yshyn-iohk]

Proposed feature

This issue is a subtask of the hyperledger/identus#16

Implement the logic for VP token validation.

Feature description

The Prover/Wallet submits a VP through the PresentationSubmission Endpoint, so the VP should be validated.

In addition to common VC/VP validation, in OID4VP, additional validations should be enforced, like Authorization Request nonce and aud. See VP token validation for more details.