Update OID4VCI implementation

Created by: nodh

Update our implementation of OpenID for Verifiable Credenital Issuance to latest draft (13, from 2024-02-08).

Far fetched goal is interoperability with EUDIW reference implementations.

The interface for actualliy issuing credentials (Issuer.issueCredential()) now needs some rework, but that should be done in a separate PR.

Fixes #47 (closed)