[ECS Managed Instances] [request]: Warm Pools for ECS Managed Instances

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**Tell us about your request**
What do you want us to build?

Warm Pools for ECS Managed Instances

**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?**

For customers who would like to use ECS MI, but need pre-provisioned instances for redundancy, or who would like to scale to zero but have faster provisioning/startup time.

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

1. Use ECS/EC2 instead with the [warm pools feature](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/using-warm-pool.html).
2. Provision an eventbridge cron dummy task to keep an MI instance always running and warm, though this is more costly than a proper warm pool.