Storage seam for by-reference OID4VCI credential offers.
An issuer stores a credential offer under an opaque id and gives the wallet
a URI containing that id. The wallet retrieves the offer through fetch/1.
Retrieval is non-consuming: the pre-authorized code inside the offer is the
single-use gate for the issuance flow.
Summary
Callbacks
Fetch a live offer without consuming it, or return :error.
Persist a credential-offer record.