Behaviour for Pushed Authorization Request storage (RFC 9126).

The store keeps normalized authorization request parameters behind a PAR request_uri reference. Values are opaque maps because the authorization endpoint still runs the normal Attesto.AuthorizationRequest validation after the reference is resolved.

The authorization endpoint uses fetch/1, not take/1, because host applications commonly establish login or consent and then re-enter the authorization endpoint with the same request_uri. Stores should expire entries by TTL; they should not consume them simply because the front channel was resolved. When :require_pushed_authorization_requests is enabled, the configured store must also implement atomic take/1; otherwise completion fails closed instead of issuing a code without a single-use claim.