[EKS] [request]: expose status of automated control plane update to DescribeCluster API

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**Tell us about your request**
From time to time, EKS control plane is patched automatically and this is [documented behavior](https://docs.aws.amazon.com/eks/latest/userguide/what-is-eks.html). But while automated update is underway, EKS does not prevent the user from applying another cluster update, which subsequently fails with a `ResourceInUseException`. Also, DescribeCluster API still shows cluster state as ACTIVE, as against UPDATING. This is misleading and leads to confusion.

**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?**
It will be useful if a new cluster state could be added to reflect automated update and should be visible on console, DescribeCluster API and describe-cluster CLI.

**Are you currently working around this issue?**
Retry UpdateCluster operation after 40 mins

**Additional context**
n/a

**Attachments**
n/a

Start by reviewing the EKS DescribeCluster API and the describe-cluster CLI behavior named in the issue, along with how the console reports cluster state. The issue does not mention repository files or tests; done would require an agreed automated-update state exposed consistently through the API, CLI, and console.

Written by the indexing model from the issue text.