Enable SSAI (server side ad insertion) traking with Event Mode

[nicolaslevy]

On behalf of the SVTA Advertising Working Group:

In previous tickets (like this, and this) we discussed how to use cover the use case of tracking SGAI with CMCD v2 Event Mode now that we report events like "enter interstitial".

In order to use CMCD v2 as a mechanism to report SSAI tracking, we should add one or two events to signal that it has occurred:

• A Period change in DASH
• A discontinuity (EXT-X-DISCONTINUITY) in HLS