Introducing feature flag management from the Vercel Toolbar

Introduces the ability to manage and override feature flags directly from the Vercel Toolbar. This integration allows developers to stay in their workflow by overriding flags from any provider (LaunchDarkly, Optimizely, Statsig, Split, Hypertune, etc.) via API Routes and script tags. The implementation involves rendering flag values via a script tag and communicating metadata through an API route. Overrides are respected by reading the `vercel-flag-overrides` cookie. This enhances QA and testing by enabling per-session overrides in local, preview, and production environments.