[ECS] [Task Definition Secrets File]: S3 File with Secrets defined with `valueFrom`

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**Tell us about your request**
ECS task definitions container definitions currently support an `environmentFile` to define environment variables. I am requesting a similar parameter `secretsFile` that allows the user to define secrets within a file. i.e. instead of the format in the file to be `KEY=VALUE`, the format can be `KEY=SECRETS_ARN` i.e. `KEY=arn::aws::secretsmanager:...`, the same way the current `key/valueFrom` works directly in the `secrets` parameter.

Another possible solution would be to add the ability to specify an entire secret manager to include, instead of each individual secret.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We reached the 64 KB task definition size limit because too many secrets were defined. This was only an issue because the `valueFrom` must be the entire ARN, which is very long. The `secretFile` will solve the issue of too many secrets, while keeping the key/values managed through secrets manager.

**Are you currently working around this issue?**
No

**Additional context**
Anything else we should know?

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