Media CDN - September 13th, 2022 [Feature]

## Feature Media CDN now supports token-based client authentication and private external origin authentication for enhanced content protection and improved interoperability with multi-CDN and multi-cloud architectures in [Preview](https://cloud.google.com/products#product-launch-stages). Token-based authentication enables signing arbitrary data (like custom user identifiers), HMAC-based cryptography, and dual token exchange for token propagation through the duration of a playback session. For more information, see [Prevent unauthorized distribution](https://cloud.google.com/media-cdn/docs/prevent-unauthorized-distribution-overview). Private external origin authentication allows Media CDN to support [private S3-compatible origins](https://cloud.google.com/media-cdn/docs/connect-to-s3-compatible-buckets) by dynamically generating signed requests with AWS Signature Version 4.