Implement OID4VCI 1.1 "interactive authorization endpoint"

The OID4VCI 1.1 draft has this new section:

https://openid.github.io/OpenID4VCI/openid-4-verifiable-credential-issuance-1_1-wg-draft.html#name-interactive-authorization-e

This flow makes OID4VCI much more similar to VCALM (but with an additional variety of endpoints to hit and session tokens, etc.). So I expect we should be able to fit it into the general exchange processing flow.