[ECR] [request]: Allowing private to private ECR pull through cache service linked role to create repositories

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**
What do you want us to build?

Currently when configuring pull through cache rule between two private ECRs the downstream repository must use Registry Permission (Scoping) or IAM Role with `ecr:CreateRepository` in order to create repository for images pulled through the cache rule. My suggestion is to add the `ecr:CreateRepository` permission into the service linked role permissions reducing 1 step required from the downstream repository in the configuration.

## Which service(s) is this request for?
Amazon Elastic Container Registry (ECR)

## Tell us about the problem you're trying to solve
**What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved?**

I want to simplify ECR pull-through cache configuration by eliminating the manual permission setup step. Currently, each downstream repository must be granted `ecr:CreateRepository` permissions, which adds configuration overhead and slows adoption of the caching feature.

**Impact of not solving this problem:**
- Additional setup complexity for pull-through cache rules
- Slower adoption due to required manual permission configuration
- Operational overhead for teams managing multiple repositories

## Are you currently working around this issue?
**How are you currently solving this problem?**

Currently we instruct downstream repository owners to grant `ecr:CreateRepository` permissions, which isn't ideal and adds overhead to adoption.

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

- This enhancement would reduce configuration overhead and improve pull-through cache adoption
- Adding `ecr:CreateRepository` to the service-linked role would be more secure than manual permission grants by keeping everything around the pull through cache permissions

Start by reviewing Amazon ECR pull-through cache rules and the service-linked role permissions described in the issue. Compare the current requirement for downstream ecr:CreateRepository permission with the requested role change, and verify that completion would remove the manual permission setup without weakening the intended access boundary.

Written by the indexing model from the issue text.