Video Analytics additions for ExperienceEvent, part 2

As a follow-up to #364, 4 properties have been broken out of PR #417 to be treated post-0.9.3 release:

* airDate
* digitalDate
* mediaSegments
* bitrateAverageBucket

See PR #433.

## What are the schemas that are affected by the issue

`/context/experienceevent.schema.json`

## What are examples of products that are impacted by the issue

Video Analytics
Video Ads
Media marketing in Target

Start by reading /context/experienceevent.schema.json and the referenced PR #433, then compare how the four properties—airDate, digitalDate, mediaSegments, and bitrateAverageBucket—are represented. Done means the affected schema includes the agreed Video Analytics additions and remains valid for the listed impacted products.

Written by the indexing model from the issue text.