[ECS][Fargate] Allow more flexible task-level CPU/memory combinations for memory-bound workloads

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Tell us about your request
Please allow more flexible task-level CPU and memory combinations in AWS Fargate so workloads with low CPU requirements but high memory needs can be right-sized without overpaying for unused CPU.

### Which service(s) is this request for?
Fargate, ECS

### Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?
In Fargate, CPU and memory must be specified at the task level using pre-approved combinations. For example, the maximum memory allowed for 0.5 vCPU is 4 GB.
This creates challenges for memory-heavy but CPU-light workloads. For instance, a Redis cache may only need 0.5 vCPU for its read-heavy traffic but require 5 GB RAM due to allocator behavior, fragmentation, and the need to provision for peak usage.
Currently, to run 5 GB RAM on Fargate, you must provision at least 1 vCPU — doubling CPU cost for no performance benefit.

The impact is forced overprovisioning, which increases costs unnecessarily and discourages running certain workloads on Fargate.

### Are you currently working around this issue?
Yes — either:

Accepting the extra cost of unused CPU by moving to a higher CPU tier, or

Switching to ECS on EC2 to get finer control over CPU/memory allocation.

### Additional context:
Redis memory optimization documentation: https://redis.io/docs/management/memory-optimization/
Fargate CPU/memory sizing documentation: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/fargate-tasks-services.html#fargate-tasks-size

Start with the Fargate CPU/memory sizing documentation and the linked Redis memory optimization documentation to understand the current limits and memory-bound use case. Done means AWS defines and supports more flexible ECS/Fargate task-level CPU/memory combinations, allowing the stated low-CPU, high-memory workload without a higher CPU tier.

Written by the indexing model from the issue text.