Investigate feature flags for KAS

This is an investigation issue for Implement feature flags for KAS (#246 - closed).

The goal is to create a proposal, to-do list and an epic for it.

• Research how feature flags for Gitaly works.
• Create a proposal on how to implement it for KAS.
• Discuss and align the implementation plan.
• Create an epic and issues.

Ref:

• Feature flags in the development of GitLab
• All feature flags in GitLab
• Use ChatOps to enable and disable feature flags