LL-HLS RENDITION-REPORT playlist variant switching

[robwalch]

This PR will...

• Use RENDITION-REPORT details to make blocking reload requests when switching level/audio/subtitle playlists
• Add type and url to "hlsSubtitleTrackSwitch" events switching to a track
  • Subtitle disabling switches to index -1 continue to only contain { id }

Why is this Pull Request needed?

These changes allow us to make blocking playlist requests for variants that should be (close to) the latest already available on the origin and edge.

Are there any points in the code the reviewer needs to double check?

These changes use the previous playlist report's LAST-MSN and LAST-PART attributes for the variant being switched to as parameters in the request for that playlist.

Improvements to low-latency switching like playlist/segment prefetching or variant latency/under-buffer estimates using these reports are not included in this change-set.

Checklist

• changes have been done against the feature/v1.0.0 branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[itsjamie]

Nice.