Draft: Protected containers: Add POST to REST API for container protection rules

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What does this MR do and why?

Adds the POST route to the REST API for container protection rules to allow the creation of new container protection rules.

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

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

🛠 with ❤ at Siemens

MR acceptance checklist

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 a new container protection rule:

curl --request POST \
  --url https://gdk.test:3443/api/v4/projects/7/registry/protection/rules \
  --header 'Authorization: Bearer ypCa3Dzb23o5nvsixwPA' \
  --header 'Content-Type: application/json' \
  --data '{
	"repository_path_pattern": "flightjs/flight-needs-to-be-a-unique-path",
	"minimum_access_level_for_push": "maintainer",
        "minimum_access_level_for_delete": "maintainer"
    }'

Related to #457518