[Batch] [request]: Enable repositoryCredentials in job definitions for Fargate compute environments

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**Tell us about your request**
Please add the `repositoryCredentials` option to Job definitions in AWS Batch similarly to how it is in ECS (https://docs.aws.amazon.com/AmazonECS/latest/userguide/private-auth.html).

**Which service(s) is this request for?**
AWS Batch

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I'm trying to use images from a private repository in Batch that's not ECR, which is impossible for Fargate compute environments. [This AWS guide](https://aws.amazon.com/blogs/compute/how-to-authenticate-private-container-registries-using-aws-batch/) notes this limitation.

**Are you currently working around this issue?**
By storing the images in a private ECR repository.

Start by reviewing the AWS Batch job definition documentation and the linked ECS private-auth guide, then compare the stated Fargate limitation with the AWS Batch private-container-registry guide. Done means AWS Batch job definitions support repositoryCredentials for images in private registries when using Fargate compute environments.

Written by the indexing model from the issue text.