Add option for Actors to delay a response

[RedKinda]

Sometimes it is useful to delay sending a response, but still allow the actor to process other messages. It can be useful in cases, where the actor needs to persist some data or make external requests, does not need a &mut self but the message response does depend on result of the persistence operation. In this case it would be possible to do something like tokio::spawn(async move { persist().await; sender.send(Ok(())) } );

This PR exposes the response oneshot sender to the actor through a new method, which actor definitions may override if they wish to. Now, I am not sure if the option to process other messages before sending a response breaks soundness or creates other issues, but I did not find any. One small issue I found is that measuring the time for metrics purposes might be misleading, if the response is actually delayed, as it measures the time for the handle() function to return, not actual sending of the response.

Let me know if there are any issues :)

[RedKinda]

Actually, I realized this is not actually great, since the handle method needs to be implemented as well, which kind of breaks it. Not sure how about fixing that