Introduce Feature Flag type Licensed

TL;DR

As part of #221053 (closed) we document feature flags in YAML.

This adds Licensed type of Feature Flag making all licensed feature flags to be explicitly defined, but still checked implicitly.

The Project#feature_available?, Namespace#feature_available? (EE), and License.feature_available? (EE) methods all implicitly check for a by default enabled feature flag with the same name as the provided argument.

Related to and they contain all prior discussions:

• !34274 (merged)
• #30228 (closed)
• !31610 (closed)

Proposal

1. We introduce new feature flag licensed and fix all of its usage
2. We require all new added feature flags of type licensed to be YAML-defined