[ECS, Fargate] [request]: Add AWS SSM Parameter Store ByPath feature

**Tell us about your request**
Add an ability to map environment variables for task containers using ByPath call.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I'm currently facing a problem when I need to handle over 70 environment variables per application container, and current implementation of AWS SSM/SM in ECS is not satisfying at all, nor the ability to use env_file directive in ecs-cli compose, because that way it exposes all secrets in task definition ¯\_(ツ)_/¯

**Are you currently working around this issue?**
Currently I'm bypassing this by using [ssm-parent](https://github.com/springload/ssm-parent) tool that pulls all my secrets during container startup using task IAM. That is fine but managing it through my ecs-params would be much better

Start by reviewing the requested ECS, Fargate, and EKS scope and the AWS SSM Parameter Store ByPath requirement. Clarify which service and task configuration should support mapping parameters to container environment variables, then define done as handling large parameter sets without exposing all secrets in the task definition.

Written by the indexing model from the issue text.