Thanks for your great job!

I think AuthLayer is not necessary. We can simply create AuthService through a constructor AuthService::new(channel: Channel, token: Option<Arc<HeaderValue>>). And then we can only depend on tower_service.

So we can just change the XxxClient::new(channel: Channel, interceptor: Option<Interceptor>) to XxxClient::new(channel: Channel, token: Option<Arc<HeaderValue>>), and create AuthService inside.

There is a simple principle that client mod can invoke rpc mod, but rpc mod can not invoke client mod. So I suggest do not define AuthService inside client, you can define it in a auth mod parallel with client.