[ECR] [request]: pull through cache repositories for Google Container Registry (now Google Artifact Registry)

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**Tell us about your request**
Please add support for dockerhub and gcr registries in this new feature:
https://aws.amazon.com/de/blogs/aws/announcing-pull-through-cache-repositories-for-amazon-elastic-container-registry/

**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?**
Most images are stored in docker hub and users are getting rate limit errors all the time...

**Are you currently working around this issue?**
Syncing these docker hub images to gcr and ecr

No files or tests are named. Start with the linked AWS announcement and the ECR request details, then determine how Docker Hub and Google Container Registry or Artifact Registry should be represented in the roadmap. Done means the requested pull-through cache support has a defined implementation scope and acceptance criteria.

Written by the indexing model from the issue text.