[EKS] [request]: Allow adding extra disks to instances when creating a self-managed node group

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**Tell us about your request**
We would like to be able to specify additional disks that should be attached to instances, during the creation of a self-managed node group.

**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?**
We have followed official instructions for creating a self-managed node group ( https://docs.aws.amazon.com/eks/latest/userguide/launch-workers.html), which suggest to use either `eksctl` or the AWS Management Console. Unfortunately, neither of them allows us to attach extra disks to the instances created.

Start with the linked EKS self-managed node group guidance, then compare the eksctl and AWS Management Console creation paths mentioned in the request. Define how additional disks should be specified and verify that a newly created node group attaches them to its instances; the issue does not name repository files or tests.

Written by the indexing model from the issue text.