Determine when draft 15 used based on issuer metadata

[TimoGlastra]

We could determine it based on the issuer metadata if:

• key_attestations_required is present (draft 15)
• Inclusion of dc+sd-jwt format identifier

Otherwise the issuer metadata is the same between draft 14 and draft 15. The main difference seems to be in the credential request structure.

As issuer we can support old and new format for the credential request, but as holder it might be a bit tricky to determine which request structure to use based on the offer and issuer metadata