CDN and fullscreen view dimensions

The beacon and stream processor now capture two additional playback-quality dimensions.

1. New cdn dimension surfaces which CDN served the manifest and segments for a view.
2. New fullscreen flag records whether the viewer watched in fullscreen at any point in the session.
3. Both fields flow end-to-end from ingestion through the analytics store.
4. They are usable as filters and breakdowns in the metrics, views and errors APIs, enabling CDN-vs-CDN quality comparisons.