Implement Covered Experience registry

The Covered Experience schema is present in the blueprint.

The registry will provide engineers with a centralized repository where to store Covered Experience Definitions.

The registry will live in the Rails app and be our source of truth.

We should add validation to guarantee that invalid data is not pushed to the registry:

• covered_experience_id should be unique
• description should not be blank
• urgency should be one of the options available in the blueprint's table
• feature_category should match with values available in stages.yml

The Covered Experience files could be stored in config/covered_experiences in the Rails app, where each yml file is a unique definition.