Add a feature flag for suggested reviewers

What does this MR do and why?

This MR adds a new feature flag named suggested_reviewers, which controls the roll-out of the suggested reviewers features on the merge request page.

Currently, there is no functionality built under this feature flag yet. This is part of a bigger epic to bring suggested reviewers to the code review experience.

Relates to #368341 (closed)

How to set up and validate locally

The feature flag can be enabled and disabled via the Rails console.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #368341 (closed)