[Fargate] [Task Resources]: Allow smaller CPU/Memory configurations

**Tell us about your request**
Allow lower CPU and Memory resource allocation for Fargate tasks.

**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?**
Fargate is very appealing because it removes the requirement to run Container Instances. However, we have a lot of microservices whose CPU and Memory requirements are lower than the minimums allowed on Fargate. Fargate is already more expensive than running on-demand instances of families like `m5` and `c5` with equivalent vCPU and Memory configurations. For tasks that require less than 0.25 vCPU and 0.5GB Memory, the cost is even higher compared to running those tasks binpacked with others on an ECS Container Instance.

**Are you currently working around this issue?**
We're using ECS with the EC2 launch type.