allow close downstream idle connection while draining

*Description*:
When stopping envoy by using `curl -X POST http://localhost:15000/drain_listeners?graceful`
Envoy only sending `Connection: CLOSE` on HTTP1, that means idle connection will be Established after `idle_timeout` at most.

*Background*:
We are building AI Gateway based on Envoy, we hope `idle_timeout` could be large enough.
Meanwhile we hope downstream idle connections could be closed as soon as possible, so that the the Gateway could be shutdown as early as possible.

[optional *Relevant Links*:]
https://www.envoyproxy.io/docs/envoy/latest/operations/admin#operations-admin-interface-drain

Maybe close downstream idle connection could be the default behaviour or add a new configuration to enable it.