[ECR] [request]: Pull through cache support for Harbor Container Registry

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**Tell us about your request**
We would like to use pull through cache with Harbor container registries with credentials.

**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 would like to use pull through cache with Harbor container registries.

**Are you currently working around this issue?**
We are currently unable to use pull through cache with Harbor registries as it is not yet supported as an upstream source registry.

No files, tests, or entry points are mentioned in the issue. Start by identifying where ECR pull-through cache upstream registries are defined and how credentials are handled; done means Harbor container registries are supported as authenticated upstream sources.

Written by the indexing model from the issue text.