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

examples/auth/client/main.go

@@ -36,6 +36,7 @@ func (r *codeReceiver) serveRedirectHandler(listener net.Listener) {
 		r.authChan <- &auth.AuthorizationResult{
 			Code:  req.URL.Query().Get("code"),
 			State: req.URL.Query().Get("state"),
+			Iss:   req.URL.Query().Get("iss"),
 		}
 		fmt.Fprint(w, "Authentication successful. You can close this window.")
 	})