[EKS] [request]: Spot & On-demand Mixed Policy for Managed Node Group

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**

Spot & On-demand Mixed Policy for 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?**

Currently our team is using the mixed-policy node group with following options.

onDemandBaseCapacity
onDemandPercentageAboveBaseCapacity

Those options give us some stability in our cluster while using spot instances to reduce the cost.

We hope to migrate our EKS cluster to utilize managed node group, but reluctant to fully moving our node-group to spot-only group. So if such options are given to Managed node group's spot type it will be grateful.

**Are you currently working around this issue?**

We are currently using basic node group to use mixed policy.

No files, tests, or implementation entry points are named; start by reading the request and comparing the existing basic node group mixed policy with EKS Managed Node Groups. Done means Managed Node Groups support equivalent on-demand base capacity and on-demand percentage settings alongside Spot capacity.

Written by the indexing model from the issue text.