[ECR] [request]: Repository deletion protection

**Tell us about your request**
Add deletion protection to ECR repositories to prevent accidental deletion when using automation or in the console.

**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?**
I'm trying to prevent a case where a production service is running in a container cluster, EKS or ECS, and something or someone wipes out a dependent repository in ECR, either by accident in the console, mis-config with automation (think Terraform), or otherwise. Preventing this would mitigate somewhat the need for additional backups or something.

**Are you currently working around this issue?**
My plan is to use a tool like `reg` (https://github.com/genuinetools/reg) to try and automate backup of layers and images to S3, so in the event of loss, our most critical images can be restored automatically within moments, rather than the mass rebuild of every image we need.

I might, if I have time try and open source a solution, that backs up a set of critical tags for a named repository to S3 using a lambda and events on ECR (haven't yet checked if there are appropriate events for this).

The issue identifies AWS ECR but no repository files, tests, or implementation entry points. Start by reviewing the ECR service capability described and the roadmap's handling of service-level requests. Done would require a defined deletion-protection feature rather than a change localized to this repository.

Written by the indexing model from the issue text.