[ECS, Fargate] [request]: Pull environment variable values from SSM

**Tell us about your request**
I want to be able to pull values for environment variables from SSM, in the same way that secrets can be pulled. The task definition would look something like this:

```json
"environment": [
{ "name": "SOME_ENVIRONMENT_VAR", "valueFrom": "/foo/some-environment-var" }
]
```

**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?**
I want to keep my config values outside of the containers, and have them pull the correct values for the environment they are running in from SSM when they boot (like they already do for secrets).

**Are you currently working around this issue?**
By using secrets for config values which are not sensitive.

Start by reviewing how ECS and Fargate currently support pulling secret values into task definitions, then compare that behavior with the requested environment.valueFrom shape and SSM path. Done means documenting or implementing an agreed approach that lets environment variables load non-sensitive SSM values at container startup, with validation of the task-definition behavior.

Written by the indexing model from the issue text.