[EKS] [request]: Add/Delete/Update AZs registered with control plane

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**Tell us about your request**

Allow users to change the AZs that are configured with an EKS Control Plane.

Currently you can now modify the subnets that are configured with the control plane but you must provide the same AZs with any changes.

**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?**
I'd really like to just be able to use data lookups for X subnets and create a cluster with it. Instead I end up needing to provide overrides/per cluster configurations on quite a few accounts if we add/remove/change subnet configurations within an account over time.

Some accounts we've configured VPCs with 4 subnets, then added 2 more. If we don't provide overrides in this case, the EKS change will fail.

Other accounts we have only 2 AZs configured and we cannot expand it to 3.

In yet another account we have an AZ configured in the VPC which is not supported by EKS so we must filter it out.

**Are you currently working around this issue?**
Maintaining manual overrides

**Additional Information**

Related: #170

Start with the requested EKS control-plane AZ changes and read the related issue #170 for context. Done means users can add, delete, or update the Availability Zones registered with an EKS control plane without maintaining manual subnet overrides; no repository files or tests are identified.

Written by the indexing model from the issue text.