Cloud CDN now supports custom named cookies and headers in the cache key, to enable A/B (multivariate) testing, canarying, and similar scenarios

## Feature Cloud CDN now supports custom named cookies and headers in the cache key, to enable A/B (multivariate) testing, canarying, and similar scenarios. Allowlisting of query parameters is now also enabled for backend buckets, to allow for cache busting. These features are available in **Preview**. For details, see the [caching documentation](https://cloud.google.com/cdn/docs/caching#cache-keys).