[Batch] [Allow setting of task size]: Allow setting task size for batch jobs

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**Tell us about your request**
What do you want us to build?

ECS has the ability to specify a task size at the task level: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task_definition_parameters.html. According to https://aws.amazon.com/blogs/containers/how-amazon-ecs-manages-cpu-and-memory-resources/, for tasks that run on EC2, if you do not set a task size, containers have full access to all CPUs on the machine. The CPUs parameter that Batch lets you set is simply a CPU reservation - placement will occur such that enough CPUs exist for each container.

In order to further isolate Batch jobs, we would like to be able to specify the task size. However that does not seem to currently be an option.

**Which service(s) is this request for?**
This could be Fargate, ECS, EKS, ECR

ECS/Batch

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved? The more details you can provide, the better we'll be able to understand and solve the problem.

**Are you currently working around this issue?**
How are you currently solving this problem?

**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.)

No repository files, tests, or implementation entry points are named. Start by reviewing the linked ECS task definition and CPU/memory resource documentation, then compare the requested task-level sizing with current ECS/Batch behavior. Done would require a defined product/API change that lets Batch jobs specify task size, but the issue does not describe an implementation path.

Written by the indexing model from the issue text.