[EKS] [request]: allow mixed architectures in managed nodegroups

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**Tell us about your request**
Please allow me to create a managed nodegroup (spot or on-demand) specifying both x86 and arm64 instance types.

**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 use a managed spot nodegroup (which works great!) but there are relatively few cost-effective arm64 options for us. Currently we use a1.xlarge and t4g.xlarge, which is helpful, but doesn't provide much resilience for available capacity - it's still common for the ASG to have problems maintaining it.

Our system and all images work perfectly on both arm64 and x86. We don't care which architecture our nodes are using - it makes no difference to us.

We want to be able to make a single spot nodegroup containing Graviton2, Intel and AMD instance families, so we have a great range of options and are very unlikely to have any capacity issues.

Although it is possible for us to make two nodegroups and just put half of the capacity in each, this doesn't really help with resilience at all, as we need the whole capacity and only having half available wouldn't be enough.

**Are you currently working around this issue?**
We're not really... just having to watch the nodegroup, swap out its instance types regularly when issues come up and switch between arm64 and x86 sometimes.

Start by reviewing the EKS managed nodegroup request and the stated constraints around spot and on-demand capacity. Confirm how mixed x86 and arm64 instance types are handled and whether the proposed capability fits managed nodegroup behavior. Done means a single managed nodegroup can specify both architectures with the requested resilience.

Written by the indexing model from the issue text.