This adds a YAML-based definition of feature flags
that are stored in `(ee/)configs/feature-flags/`.

Currently none of feature flag types are required
to have a present YAML definition.

This definition contains information like:

- what MR introduced FF
- helps to create and has a issue that tracks
  the rollout and removal of FF
- intentionally adds `default_enabled` to YAML
- force checks the consistency of `default_enabled`
  between what is in code and what is in YAML