Extend Packages Project policy to consider access level

What does this MR do and why?

This MR is the next step of the implementation plan in #329253 (closed).

In !82808 (merged), we added a new Package Registry visiblity setting to the project settings (behind a feature flag). This MR extends the Packages::Policies::ProjectPolicy policy to consider the new package_registry_access_level setting.

🛠 with ❤ at Siemens

/cc @bufferoverflow

How to set up and validate locally

1. Enable the feature flag:

   Feature.enable(:package_registry_access_level)

2. Change the package_registry_access_level of a private project to allow access for everyone:

   project = Project.find(2)
   project.project_feature.update!(package_registry_access_level: ProjectFeature::PUBLIC)

3. Check if policy allows :read_package permission for anonymous user (nil):

   Packages::Policies::ProjectPolicy.new(nil, project.packages_policy_subject).allowed?(:read_package)

MR acceptance checklist

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