[eks] [request]: Support logical grouping of MNGs for updates

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**Tell us about your request**
I'd like to be able to logically group multiple MNGs together to be treated as a single unit when it comes to updates; this would match the guidance to create a MNG per AZ when running stateful workloads. Alternatively if MNGs could create an ASG per AZ behind the scenes and manage them as one this would also work. For a managed service I shouldn't have to think about this.

**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?**
When I update my MNGs an instance per group is terminated which can causes a race condition where nodes can't terminate due to PDBs and pods being spread across multiple nodes. This is especially noticeable when running a node per AZ with stateful workloads. When this occurs not only does it disrupt the services but it locks some of them up due to the PVs becoming orphaned.

**Are you currently working around this issue?**
I can't use MNGs while this behaviour isn't supported.

**Additional context**
See #1866

**Attachments**
n/a

Start by reviewing issue #1866 and the EKS managed node group update behavior described here. The issue does not name implementation files or tests; done would require an agreed design for grouping MNGs or managing per-AZ ASGs and a way to verify update behavior without the reported disruption.

Written by the indexing model from the issue text.