Stop task on signal

[yonran]

feat: Include fractional seconds in the log messages

feat: Use Session to create Credentials (to support more credential resolutions such as sso_account_id in ~/.aws/config)

feat: Stop task on timeout or signal. Previously, when the --timeout was hit or ecs-task was interrupted, it exited immediately without cleaning anything up. This commit makes ecs-task call StopTask, followed by waiting up to 1 minute for the task to complete, followed by waiting 10 seconds for any additional logs.