[ECS] [request]: support cross account secrets for Private registery authentication for tasks

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**Tell us about your request**
Support cross-account secrets for Private registry authentication for tasks

**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?**
According to:
https://docs.aws.amazon.com/AmazonECS/latest/developerguide/private-auth.html

"The secret you reference can be from a different Region than the task using it, but must be from **within the same account**."

We store the secrets in a different account and we have built automation to rotate these secrets.
If we need to bring the secret to the same account as ECS we are making the rotating process more complex.

**Are you currently working around this issue?**
We don't have a workaround. We are forced to change our secret rotation automation to enable teams to use the external container registrey in our environment.

The request concerns ECS and Fargate private registry authentication and links the AWS private-auth documentation; start by reading that documentation and checking the repository for any ECS task-definition or secrets-related entry points. No repository files or tests are named, and done would mean supporting a secret from another account for task registry authentication.

Written by the indexing model from the issue text.