Introduce feature flag definition (RUN AS-IF-FOSS)

What does this MR do?

Related to: #30228 (closed)

Resolves: #221053 (closed)

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 does not change yet workflow.

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

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Style guides