feat(eks): support MixedInstancePolicy for self-managed nodes

[chancepants]

Adds an optional parameter to the cluster addAutoScalingGroupCapacity method to support MixedInstancePolicy. Currently the only alternative is to heavily utilize escape hatches to attach a MixedInstancePolicy to the ASG created within this method.

Issue # (if applicable)

Closes #35732

Reason for this change

Users of eks cluster.addAutoScalingGroupCapcity do not have a convenient way to apply a MixedInstancePolicy to their ASG.

Description of changes

Added an optional parameter to cluster.addAutoScalingGroupCapcity to allow users to provide a MixedInstancePolicy which is applied to the ASG.

Description of how you validated changes

I added unit tests which largely cover the validation logic and added a unittest to verify CDK deployments succeed with the new parameter.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
Please try merge from main to avoid findings unrelated to the PR.

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	Tests	Passed ☑️	Skipped	Failed ❌️
Security Guardian Results	96 ran	95 passed		1 failed

Test	Result
Security Guardian Results
packages/@aws-cdk-testing/framework-integ/test/aws-eks/test/integ.eks-cluster-mixed-instances.js.snapshot/aws-cdk-eks-cluster-mixed-instances-test.template.json
iam-no-world-accessible-trust-policy.guard	❌ failure

• View Security Guardian Results

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
Please try merge from main to avoid findings unrelated to the PR.

---

	Tests	Passed ☑️	Skipped	Failed ❌️
Security Guardian Results with resolved templates	96 ran	93 passed		3 failed

Test	Result
Security Guardian Results with resolved templates
packages/@aws-cdk-testing/framework-integ/test/aws-eks/test/integ.eks-cluster-mixed-instances.js.snapshot/aws-cdk-eks-cluster-mixed-instances-test.template.json
codepipeline-cross-account-role-trust-scope.guard	❌ failure
guardhooks-no-root-principals-except-kms-secrets.guard	❌ failure
iam-role-no-broad-principals.guard	❌ failure

• View Security Guardian Results with resolved templates

[aws-cdk-automation]

This PR has been in the MERGE CONFLICTS state for 3 weeks, and looks abandoned. Note that PRs with failing linting check or builds are not reviewed, please ensure your build is passing

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