Cloud CDN support for custom named cookies and headers in the cache key are Generally Available

## Feature Cloud CDN support for custom named cookies and headers in the cache key are [Generally Available](https://cloud.google.com/products/#general-availability). You can use these features for A/B (multivariate) testing, canary testing, and similar scenarios. Allowlisting of query parameters is now also enabled for backend buckets, to allow for cache busting. For details, see the [caching documentation](https://cloud.google.com/cdn/docs/caching#cache-keys).