Protected containers: Add GET to REST API for container protection rules

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What does this MR do and why?

Adds a GET route to the REST API to retrieve a list of container repository protection rules.

This is part of #457518 (closed) and still behind a feature flag.

The MR is mostly equivalent to the MR that implements the GET request for package protection rules.

🛠 with ❤ at Siemens

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

MR Checklist ( @nwittstruck)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

How to set up and validate locally

1. Enable feature flag via rails c Feature.enable(:container_registry_protected_containers)
2. Create some protection rules:

15.times { |i| ContainerRegistry::Protection::Rule.create(project: Project.find(7), repository_path_pattern: "flightjs/flight#{i}", minimum_access_level_for_push: :maintainer, minimum_access_level_for_delete: :maintainer) }

3. Use curl to fetch a list of container protection rules:

curl -k \
  --url https://gdk.test:3443/api/v4/projects/7/registry/protection/rules \
  --header 'Authorization: Bearer ypCa3Dzb23o5nvsixwPA'