Support istio in EKS add-ons

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**Tell us about your request**
Support istio in EKS add ons.

**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?**
With this issue solved, it's easy to build a cluster in which we can use istio as a service mesh.
This is useful e.g. for routing and in-transit encryption between all pod communications.

**Are you currently working around this issue?**
I'm manually deploying istio right now.

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

The issue names EKS add-ons and Istio but no repository files, tests, or entry points. Start by locating the EKS add-ons implementation or roadmap area and reviewing how add-ons are currently supported. Done means the scope and validation for supporting Istio as an EKS add-on are defined and implemented.

Written by the indexing model from the issue text.