[ECR] [request]: Pull through cache for self-hosted Gitlab instances

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**Tell us about your request**
Currently, AWS ECR supports authenticated pull through cache for registry.gitlab.com, however it **_does not_** support self-hosted Gitlab instances. For customers who self-host Gitlab, using ECR as a pull through cache is impossible.

**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?**
For security and compliance reasons, we prefer to give clients access to our ECR registry, rather than our self-hosted Gitlab registry.

**Are you currently working around this issue?**
Currently, we have to maintain custom automation to push new images into ECR and to create ECR repositories. We would like to have the features of an ECR pull through cache connected to a self-hosted Gitlab instance.

**Additional context**
In general, I don't understand why AWS restricts pull-through cache to specific hosts. As far as I can tell, as long as a registry is conforming to the [oci distribution spec](https://github.com/opencontainers/distribution-spec/blob/main/spec.md), AWS should have a standard interface for authenticating and pulling images from a vast array of registries.

No repository files, tests, or implementation entry points are identified. Start by reviewing the ECR pull-through cache behavior described in the request and the linked OCI Distribution Specification, then determine what support for authenticated self-hosted GitLab registries would need to cover and how completion could be verified.

Written by the indexing model from the issue text.