OID4VCI by-reference Credential Offer endpoint (draft-ietf-oauth-openid4vci §4.1.3).

Serves the stored Credential Offer object a wallet dereferences from a credential_offer_uri. The offer is created and stored elsewhere - by the host, through Attesto.CredentialOfferStore.put/1 - when it hands the wallet the offer link; this endpoint only serves the document back, non-consuming, until it expires or is unknown.