[ECS] [request]: Editable CapacityProviderReservation AlarmLow duration (is always 15min)

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**Tell us about your request**
EC2 scale in via ECS Capacity Provider is too slow ue to the hard coded 15min alarm-low.
I wish I could change it to 5 or 10 min.

**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?**
I run tasks that execute jobs stored in a SQS.
A job takes few minutes to execute.
Most of the time, there is no job at all ; 1 job will be created randomly ; sometimes N jobs will be created at once.
When all jobs are finished, I want to scale in quite fast because it is likely there won't be new job in the next few minutes.

CAS automatically creates a "CapacityProviderReservation AlarmLow" that alarms after 15min. Scale in is then very slow, compared to a few minutes job.

Fargate would be an option, but a GPU is required.

**Are you currently working around this issue?**
I don't, I pay for idling EC2 instances.