Add inactivity timeout support

[zenazn]

Some infrastructure providers provide scale-to-zero support: the provider will automatically start your service just-in-time when a request arrives at a minor latency hit, and will stop your application if there are no in-flight connections. This helps save money, because you aren't billed for the time in which your application is idle.

This works best when the application cooperates, voluntarily exiting after a period of inactivity (especially for a proxy like sing-box which has long-lived persistent connections). This PR implements this cooperation for sing-box: it watches for traffic and keeps a last-activity timestamp. Every 10 seconds, it checks if there has been any activity in a configured timeout. If not, it calls os.Exit(0).

I've tested this out on my scale-to-zero platform of choice and it seems to work well! I'm seeing a substantial reduction in costs without much degradation in user experience using a timeout of 1m. Of course, sing-box behaves exactly as it previously did when this option is not enabled.

[zenazn]

@nekohasekai is there any interest in merging this? It would be a big help for me, but I understand if it doesn't seem helpful enough for a more general audience. If you're not interested, I'll close this PR and maintain my own patches—please let me know!