[Feat]: cull_idle_timeout mechanism requirement

## What is your suggestion?
Maybe cull_idle_timeout mechanism should be provided. If there hasn't been an active connection to code-server in X minutes, we should exit with status 0. It has been mentioned in #1636 @nhooyr

## Why do you want this feature?
I am working on develop online VSCode support by code-server. To enhance the resource utilization rate, I require cull-idle-timeout mechanism. If users do not use for a period of time, I hope the container killed. According to the doc, code-server provides the heartbeat mechanism. If I keep the WebPage on but do nothing, the time of heartbeat will be updated every minute. This is not what I want.

## Others
I have seen that such feature is referred like issue #1636. However, I failed to launch with --idle-timeout args. I want to confirm if such function is supported right now? If not, is there any methods to support my requirement? I'll appreciate it if someone could help.