[Fargate] [request]: Burstable CPU

### Community Note

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**Tell us about your request**
Similar to how t1/t2/t3 instances work, you are allocated a portion of the CPU but can burst to 100%.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I've migrated a few lower-traffic services that were running t2 instances. I didn't think about it at the time, but 25% of a CPU core 100% of the time is a lot different than 100% of a CPU core 25% of the time. For more CPU heavy endpoints my latency increased substantially and brought the service to a crawl.

**Are you currently working around this issue?**
Paying for a full CPU core :(

**Additional context**
This was originally requested [here](https://github.com/aws/containers-roadmap/issues/163), but an entirely different thing was delivered and the request closed asking for a new request to be opened for the original request!