[EKS] [Feature]: Allow custom KMS keys to encrypt underlying etcd volumes

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

**Tell us about the problem you're trying to solve**

Currently, the etcd Nodes for EKS are encrypted using AWS controlled KMS keys, as per the below snippet from the documentation:

"All of the data stored by the etcd nodes and associated Amazon EBS volumes is encrypted. Amazon EKS uses master encryption keys that generate volume encryption keys which are managed by the Amazon EKS service." - https://docs.aws.amazon.com/en_us/eks/latest/userguide/clusters.html

However, some users may prefer to implement their own KMS keys for compliance purposes. It seems this currently isn't possible in EKS. This would be a useful feature for users who are interested in adopting EKS but require control over encryption keys for compliance reasons.

Perhaps we could allow users to specify a KMS key as a parameter for CreateCluster() ?

This is a roadmap request, and the issue names no repository files or tests. Start by reviewing the EKS CreateCluster API and the linked EKS cluster-encryption documentation. Done would mean defining and supporting a user-provided KMS key for encrypting the underlying etcd volumes.

Written by the indexing model from the issue text.