[Fargate] [request]: Support use of swap memory

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**Tell us about your request**
Support configurable swap for Fargate containers & tasks. Use cases are similar to #322. (Note there are a number of requests in that issue for Fargate swap support.)

**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 have a long-running container with one periodically-run process that requires substantially more memory than other processes. I don't care about CPU performance for this process. So it is an ideal candidate for swap.

**Are you currently working around this issue?**
By running the Fargate task configured with the worst-case memory requirement. Before addition of the memory-intensive process, I could run with container memory = 1024. Now, due to this one periodic process, I must run with container memory = 3072. Doing so consumes more memory resources (and associated expense) than is necessary.

**Additional context**
According to the [documentation](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/container-swap.html):

> The swap space container definition parameters are only supported for task definitions that specify the EC2 launch type. These parameters are not supported for task definitions intended only for Amazon ECS on Fargate use.

**Attachments**
n/a

Start by reviewing the issue's Fargate context and the linked Amazon ECS container swap documentation, then compare the current EC2-only swap parameters with Fargate task definitions. Define what configurable swap should mean for Fargate containers and tasks, including the memory and cost behavior described here; the issue names no repository files or tests.

Written by the indexing model from the issue text.