[ECS] [request]: Use Fargate spot from Step Functions RunTask

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**Tell us about your request**
When we use ECS RunTask API from Step Functions ([doc](https://docs.aws.amazon.com/step-functions/latest/dg/connect-ecs.html)), we want to set `capacityProviderStrategy` to use Fargate spot capacity.

**Which service(s) is this request for?**
Fargate, ECS, Step Functions

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

Fargate spot is much cheaper than on-demand capacity. Also with Step Functions we can handle spot interruption events more easily. It'd be great match Step Functions x Fargate spot.

**Are you currently working around this issue?**
Currently we can only use fargate on-demand capacity.

**Additional context**

**Attachments**