ECS exec: user can cancel long-running commands

[justinmk3]

Problem

If ecs exec is used to run a command that takes too long or hangs, Toolkit does not provide a way to cancel it.

https://github.com/aws/aws-toolkit-vscode/issues/2607#issuecomment-1210098083 :

if you accidentally run a continuous command there's no way to stop it other than stopping the task.

ref #2607

Proposal

• Surface ecs exec tasks via a command and/or in AWS Explorer.
• User can cancel tasks that were started from the Toolkit ecs exec feature.