Draft: feat: use gitlab feature flag

What does this merge request do and why?

This MR introduces the feature flag system in AI Gateway. We use GitLab Feature Flag which is powered by Unleash SDK.

You can control the feature flags in the GitLab Feature Flag UI. e.g.:

This way, we can gradually rollout a critical change to production.

Closes #555 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.