Add feature categories as an external ref to the JSON schema

Spawned from discussion here.

We should use an external reference to validate a feature category. This will make sure that Covered Experiences won't be pushed to the repository with invalid values. Plus, we don't need to keep the values up-to-date within Labkit, as it will be fetched from the source.

The Rails app has always an up-to-date version in https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/feature_categories.yml -- the fact it is a plain list makes it suitable for consumption as-is.

We could also consider stages.yml, as it is the source of truth, however, it requires extracting the values from a nested structure.