[ECR] [request]: Cross Partition Replication for Repositories

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**Tell us about your request**
What do you want us to build?
Cross [partition](https://docs.aws.amazon.com/general/latest/gr/aws-arns-and-namespaces.html) replication for images and tags in ECR Repositories

**Which service(s) is this request for?**
ECR

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We are trying to replicate container images from the "standard" partition to other partitions in AWS (govcloud, china, etc)

**Are you currently working around this issue?**
How are you currently solving this problem?
Grabbing standard partition IAM creds, pulling image locally, switching IAM roles to a different partition, pushing the image to that partition.

**Additional context**
Anything else we should know?
EC2 has recently rolled out support for copying AMI images across partitions: https://aws.amazon.com/about-aws/whats-new/2021/04/amazon-ec2-allows-copy-amazon-machine-images-across-aws-govcloud-aws-china-other-regions/

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

No repository files, tests, or entry points are identified. Start by reviewing the linked AWS partition documentation and the referenced EC2 cross-partition AMI announcement, then define what ECR support would need to cover; done means replicating container images and tags from the standard partition to GovCloud and China partitions.

Written by the indexing model from the issue text.