[ECS] [ECS Exec idle timeout]: ability to set the idle timeout value

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**Tell us about your request**

I would like to request the ability to set the idle timeout time of an ECS Exec session as a parameter. Currently, [the timeout is set at 20 minutes](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ecs-exec.html#ecs-exec-considerations), which can be limiting for certain use cases.

**Which service(s) is this request for?**
ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

Having the option to adjust this timeout would greatly improve the flexibility and functionality of the ECS Exec service. Thank you for considering this feature request.

**Are you currently working around this issue?**

Reconnecting after the default idle timeout

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Start by reading the linked ECS Exec considerations documentation and confirm how the current 20-minute idle timeout applies. Determine where ECS Exec session parameters are documented and configured. Done means users can set an idle-timeout value for a session and the session honors it instead of the default.

Written by the indexing model from the issue text.