[WIP] Implement Alternative MPDs and List MPDs

[sebastianpiq]

This PR starts implementing alternative MPDs and list MPDs.

Summary

• Added parsing logic for events to retrieve alternative MPD events.
• Added logic to the event controller to recognize alternative MPD events.
• Added parsing of Periods with the mpdLink value.

Test cases

Assets used to test the different cases

Alternative MPDs:

• Asset case 1 (Simple Alternative MPD):
  https://comcast-dash-6-assets.s3.us-east-2.amazonaws.com/AlternativeMPDs/case1.mpd

List MPDs:

• Asset case 1 (Simple List MPD):
  https://comcast-dash-6-assets.s3.us-east-2.amazonaws.com/ListMPDs/case1.mpd