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

conformance/everything-client/main.go

@@ -66,6 +66,11 @@ func init() {
 		"auth/token-endpoint-auth-basic",
 		"auth/token-endpoint-auth-post",
 		"auth/token-endpoint-auth-none",
+		"auth/iss-supported",
+		"auth/iss-not-advertised",
+		"auth/iss-supported-missing",
+		"auth/iss-wrong-issuer",
+		"auth/iss-unexpected",
 	}
 	for _, scenario := range authScenarios {
 		registerScenario(scenario, runAuthClient)
@@ -232,6 +237,7 @@ func fetchAuthorizationCodeAndState(ctx context.Context, args *auth.Authorizatio
 	return &auth.AuthorizationResult{
 		Code:  locURL.Query().Get("code"),
 		State: locURL.Query().Get("state"),
+		Iss:   locURL.Query().Get("iss"),
 	}, nil
 }