[ECR] [request]: Repository configuration defaults

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**Tell us about your request**

Repository configuration defaults (e.g. Turn on scan-on-push by default)

**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?**

Since ECR scan on push was announced, customers have been asking their internal teams to enable this feature for new and existing repos, and realizing there are hundreds of opportunities to turn this on, but lacking a good way to guide teams to do this automatically going forward.
Could this be an option, to turn on scan-on-push by default?

**Are you currently working around this issue?**

Can think of a periodic lambda, but not an ideal solution.