[ECR] [request]: EventBridge event on new repository creation

### Community Note

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**Tell us about your request**
Add a repository-created Event to EventBridge.

**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?**
When colleagues add a new ECR repository they need to remember to run a Lambda that will grant sharing with our test/prod accounts. It would be better if they did not need to know - instead the lambda could be triggered by a new repository event and just set the permissions automatically.

**Are you currently working around this issue?**
Manually run Lambda. Indeed it runs at 8am every day to ensure permissions but it's not ideal.

Start with the ECR request and the current manual Lambda workaround described in the issue. Verify the expected repository-created event, its EventBridge availability, and how it would support automatic permission setup; done means the event reliably enables that automation.

Written by the indexing model from the issue text.