[EKS][Fargate][request]: Make it possible to select EC2 instance type in Fargate

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**Tell us about your request**
We would like to be able to specify which EC2 instance type to use in Fargate.

**Which service(s) is this request for?**
Fargate in EKS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We are using Fargate for a computationally heavy application with unpredictable loads. Having good and predictable performance is very important for our use case. We see differences in performance, which we suspect are due being assigned different EC2 instance types in Fargate. We have seen cases being 30% slower in some instances.

**Are you currently working around this issue?**
We might need to look into setting up our own EC2 instances and handle scaling.

**Additional context**
Anything else we should know?

**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.)

Start by reviewing the request and its EKS Fargate context; the payload names no implementation files, tests, or entry points. Done means establishing whether and how users can select an EC2 instance type for Fargate, with the required scope clarified before implementation.

Written by the indexing model from the issue text.