[EKS]: support in-place update of EncryptionConfig in CloudFormation

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**Tell us about your request**

EKS has supported adding a KMS key to an existing cluster to encrypt etcd secrets since at least March 2021 (https://aws.amazon.com/about-aws/whats-new/2021/03/amazon-eks-supports-adding-kms-envelope-encryption-to-existing-clusters/).

Unfortunately, CloudFormation has not yet gained support for this feature, and will try to replace the cluster if `EncryptionConfig` is added (https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-eks-cluster.html).

**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?**
Supporting this directly in CloudFormation would allow for much simpler maintenance/management of the cluster, and would allow for the cluster to be fully described in the template.

**Are you currently working around this issue?**
I have had to handle these updates out of band.

Start with the linked AWS announcement and CloudFormation EKS cluster resource documentation to understand the requested in-place EncryptionConfig update. No repository files or tests are named; done means CloudFormation supports adding EncryptionConfig to an existing EKS cluster without replacing it.

Written by the indexing model from the issue text.