Introducing feature flag management from the Vercel Toolbar

What this post added

Introduces the ability to override feature flags directly from the Vercel Toolbar, integrating with external providers (LaunchDarkly, Optimizely, Statsig, Hypertune, Split) and custom setups. This feature allows for session-specific flag overrides to improve QA and development workflows. Implementation involves rendering flag values via a script tag (e.g., using `@vercel/flags/react`) and defining flag metadata via an API route (e.g., `.well-known/vercel/flags/route.ts`). Overrides are respected by reading the `vercel-flag-overrides` cookie.