Introduce Feature Flag type Ops

TL;DR

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

This adds Ops type of Feature Flag.

Related to and they contain all prior discussions:

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

Proposal

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