[EKS] [request]: add support for nested virtualization for m8i, c8i, r8i instances in eks nodegroups

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**Tell us about your request**
The recent release of C8i, M8i, and R8i instance types introduced support for nested virtualization on non-bare-metal instances. However, this capability is not exposed through EKS managed nodegroups. Nested virtualization is disabled by default when creating a managed nodegroup, ~~and attempts to enable it via a launch template's CPU options are silently ignored~~ and the resulting instances are provisioned without nested virtualization enabled.

**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 operate multi-tenant clusters where workloads require strong isolation, which we enforce using Kata Containers. Because Kata depends on nested virtualization, we have been constrained to bare-metal instances, which carry significant cost overhead.
Supporting nested virtualization on non-bare-metal instance types such as C8i, M8i, and R8i would allow us to meet the same isolation requirements at lower cost.

**Are you currently working around this issue?**
No workaround has been identified at this time; we are currently exploring options.

Start by researching EKS managed nodegroups and the launch template CPU options mentioned in the issue, then verify how nested virtualization is configured for C8i, M8i, and R8i instances. Done means EKS managed nodegroups expose and enable nested virtualization for these non-bare-metal instance types, with the behavior documented and validated.

Written by the indexing model from the issue text.