Add a root-namespace feature flag for controlling rate limits

What does this MR do and why?

Adds a FF specific rate-limit bypass for selected namespaces

References

https://gitlab.com/gitlab-com/request-for-help/-/issues/3227

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.