Protected packages: REST API GET package protection rules

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What does this MR do and why?

Protected packages: REST API GET package protection rules

Adds a GET route to the REST API to get a list of package protection rules for a project.

This is part of #457512 and still behind a feature flag.

🛠 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

Numbered steps to set up and validate the change are strongly suggested.

1. Enable feature flag via rails c

Feature.enable(:packages_protected_packages)

2. Create a package protection rule that we will get in the next step

Packages::Protection::Rule.create(
  project: Project.find(7),
  package_type: :npm,
  push_protected_up_to_access_level: Gitlab::Access::DEVELOPER,
  package_name_pattern: "@gitlab-org/npm-package-new-enum-*"
)

3. Now, you should be able to get a list of package protection rules with the REST api:

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