OAuth 2.0 RAR Metadata and Error Signaling

OAuth 2.0 Rich Authorization Requests (RAR), as defined in [RFC9396], enables fine-grained authorization requests, using structured JSON objects. While RAR [RFC9396] standardizes the exchange and processing of authorization details, it does not specify metadata describing authorization details types. This document defines a machine-readable metadata format for authorization servers to provide authorization details type documentation including JSON Schema [JSON.Schema] definitions, as well as interoperable discovery via OAuth Resource Server Metadata [RFC9728]. It also defines a new WWW-Authenticate normative OAuth error code, insufficient_authorization_details, enabling resource servers to indicate inadequate authorization details as the cause of failure, as well as an OPTIONAL response body which MAY be returned alongside the insufficient_authorization_details error, providing an informative yet actionable authorization details object, which can be used directly in a subsequent OAuth request.