feat: add idle timeout

[vvcephei]

Description

Expose a config to set the server's idle timeout to work around #6970

Testing done

Set the configuration to 5 seconds, ran the server locally, and verified that when I create a transient query on the HTTP2 API, the connection gets closed after 5 seconds of inactivity. I also verified that if I cancel my query (Ctrl-C on my curl command), the transient query gets shut down on the server.

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[ghost]

@confluentinc It looks like @vvcephei just signed our Contributor License Agreement. 👍

Always at your service,

clabot

[agavra]

nice - I think having this config also gives us a workaround for customers that need it. I think we should have this regardless of whether or not we end up doing #7555

if we are concerned with 24h, we can start the default at 1h and then allow customers to increase it.

[guozhangwang]

LGTM, thanks @vvcephei

[guozhangwang]

if we are concerned with 24h, we can start the default at 1h and then allow customers to increase it.

Personally I think 24h is okay :)