[EKS] [request]: EKS Alpha Clusters

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**

Support Alpha clusters similar to GKE

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

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

Many users want to enable alpha level feature gates on the control plane. Allowing Alpha clusters where AWS simply enables every feature gate would allow testing/development in a managed kubernetes provider of alpha features before they hit Beta/GA.

Many users have requested that EKS support expose feature gates that users can enable, but this comes with stability concerns. This might be a decent tradeoff.

#512
#2314
#978

I also assume this would be useful internally for the EKS Team while validating/testing new Kubernetes versions.

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

Users would need to run their own control plane or use GKE.

**Additional context**

GKE has supported this for a while.

Start with this issue and the related issues #512, #2314, and #978. Compare the requested EKS Alpha cluster behavior with the GKE capability described here, including the stability concerns around enabling all control-plane feature gates. Done would mean an agreed and implementable approach for supporting Alpha clusters in EKS.

Written by the indexing model from the issue text.