Expose graceful stop timeout

[macournoyer]

A timeout is used here: https://github.com/socketry/async-container/blob/6805b6b0469aa2b4e81256f324445c767407220e/lib/async/container/group.rb#L124-L145. But, it is not configurable anywhere.

This means gracefully stopping a server will wait only 1 second before killing workers.

Also, I think the default of 1 second should be bumped. Perhaps 10 seconds?

For reference, by default:

• Unicorn will wait 60 seconds before sending SIGKILL
• Puma will wait forever

[trevorturk]

👋 just wanted to chime in to say this would be nice to have -- I'm wanting to set a graceful-stop timeout of 30 seconds on Heroku to better handle auto-scaling