[ECR] [request]: Registry configuration to reject requests without a digest

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**Tell us about your request**
Introduce a configuration on for each ECR registry to reject requests without a digest.

**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?**
Whilst immutable tags are great, to encourage everyone to use digests image in helm charts / etc.
I would like the option to enforce that at repository level.

**Are you currently working around this issue?**
No

**Additional context**
Best practices is to use digest for images, to ensure that they can't be swapped.

**Attachments**

No source files, tests, or entry points are identified in the issue. Start by reviewing how AWS ECR registry and repository configuration is represented, then define the configuration scope and request behavior; done means a repository-level option can reject image requests that do not use a digest.

Written by the indexing model from the issue text.