[ECR] [request]: Allow to use different image scanners in ECR native container image scanning

**Tell us about your request**
What do you want us to build?

Allow to use other backends to do image scanning, not only Clair.

**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?**
What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved? The more details you can provide, the better we'll be able to understand and solve the problem.

There are several image scanning solutions and would be nice to allow to ECR to scan the images just relying on other scanners, or using several of them.

**Additional context**
Anything else we should know?

Yes, we talked previously about this with @mhausenblas and told about tagging him here.

Thank you!

No files, tests, or entry points are mentioned. Start by reviewing the ECR native image-scanning design and the existing Clair integration, then compare the requested alternative or multiple scanner backends. Done means agreeing on a supported integration model and implementing the necessary ECR behavior with coverage for scanner selection or combination.

Written by the indexing model from the issue text.