Add DigitalCredentialIssuanceResponse partial

[timcappalli]

I find the most confusing part is the return of DigitalCredential interface, while the members make sense protocol and data, the fact that a DigitalCredential is returned is confusing. Maybe we should create another interface e.g. DigitalCredentialIssuanceResponse that also inherits Credential? This is only slightly better though because if the Credential inheritance.

Originally posted by @mohamedamir in https://github.com/w3c-fedid/digital-credentials/issues/343#issuecomment-3200827143