[EKS/Fargate] [request]: Support for pod anti-affinity on fargate

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**Tell us about your request**
EKS fargate used in combination with the pod anti-affinity feature does not seem to work as expected. Potentially this is because the fargate provisioned nodes are only added to the cluster at the time of scheduling?

**Which service(s) is this request for?**
EKS on fargate

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I am trying to deploy an application with high availability over 2 AZ's on Fargate. I tried using the PodTopologySpread feature, but it seems this is currently not supported either:
https://github.com/aws/containers-roadmap/issues/824
Am I right to conclude that you currently cannot deploy an application highly available over multiple AZ's on fargate and keep it balanced after scale out/in events?

**Are you currently working around this issue?**
I currently have not found a workaround for Fargate.

Start by reviewing the EKS on Fargate scheduling behavior described in the issue and the related PodTopologySpread issue #824. Investigate how pod anti-affinity behaves across availability zones during scale-out and scale-in events. Done means the requested anti-affinity behavior is supported on Fargate and remains balanced across two AZs.

Written by the indexing model from the issue text.