[EKS] Enable HPA with CloudWatch metrics and alarms

**Tell us about your request**
Many ECS customers make use of ECS service autoscaling based on CloudWatch metrics and alarms. This functionality is desired in EKS.

Community projects that add this support include https://github.com/chankh/k8s-cloudwatch-adapter

**Which service(s) is this request for?**
This could be EKS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We need to be able to use HPA to autoscale pods based on Cloudwatch metrics and alarms.

**Are you currently working around this issue?**
We use ECS.

Start by reviewing the EKS HPA documentation and the linked k8s-cloudwatch-adapter project to understand how CloudWatch metrics and alarms could feed Kubernetes autoscaling. Define the supported integration and its operational requirements before identifying the relevant AWS containers components. Done means EKS users can configure HPA scaling from CloudWatch metrics and alarms.

Written by the indexing model from the issue text.