auth: issuer mix-up mitigation (#859)

This PR functions as a Reference implementation of
[SEP-2468](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2468)
/ [RFC9207](https://datatracker.ietf.org/doc/rfc9207/).

This PR hardens the MCP OAuth Client functionality against Mix-Up
attacks:
>   Mix-up attacks aim to steal an authorization code or access token by
>   tricking the client into sending the authorization code or access
> token to the attacker instead of the honest authorization or resource
>   server

This PR hardens the client by adding support for a new `iss` parameter
in authorization responses:
- Authorization Servers broadcast support for the `iss` parameter via
the `authorization_response_iss_parameter_supported` metadata parameter
- If the parameter is supported, clients expect to receive the `iss`
parameter in the authorization response
- Clients compare the `iss` parameter in the authorization response to
the `Issuer` parameter in the authorization metadata. The two must match
exactly for the response to be processed.

Fixes #941

oauthex/auth_meta.go

@@ -114,6 +114,13 @@ type AuthServerMeta struct {
 	// ClientIDMetadataDocumentSupported is a boolean indicating whether the authorization server
 	// supports client ID metadata documents.
 	ClientIDMetadataDocumentSupported bool `json:"client_id_metadata_document_supported,omitempty"`
+
+	// AuthorizationResponseIssParameterSupported indicates whether the authorization server
+	// provides the "iss" parameter in authorization responses per [RFC 9207].
+	// When true, clients must verify the "iss" parameter is present and matches the Issuer field.
+	//
+	// [RFC 9207]: https://www.rfc-editor.org/rfc/rfc9207
+	AuthorizationResponseIssParameterSupported bool `json:"authorization_response_iss_parameter_supported,omitempty"`
 }
 
 // GetAuthServerMeta issues a GET request to retrieve authorization server metadata