[ECR] [Cross region replication of existing images]: Feature that allows automatic replication of existing images without update

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**Tell us about your request**
What do you want us to build?
Currently the ECR cross region replication (CRR) feature only replicates images and repositories when an update is made after enabling CRR. This means that images that do not get updated will never be replicated, even if this is wanted. I'm requesting a feature that will give the option of replicating all repositories and images in the specified registry, or some kind of filter which allows a desired list of existing repositories/images to be replicated without having to update them.

**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?**
I'm trying to replicate an ECR registry into another region for availability/disaster recovery purposes, but the images which have not/will not be updated since cross region replication will not replicate until an update. We have a huge amount of repos and images within them, and it will be a tedious and very manual process to update each of them in a way that will force the replication to take place. Would be much easier to have an additional option that allows this to be done automatically.

**Are you currently working around this issue?**
Manually updating images to force through the replication.

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

**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 files, tests, or implementation entry points are mentioned. Start by reviewing Amazon ECR cross-region replication behavior and the requested existing-image scope; done means an agreed implementation that can replicate selected or all existing repositories and images without requiring image updates.

Written by the indexing model from the issue text.