Implement support for LL-HLS #EXT-X-PRELOAD-HINT part loading

Adding support for Hinted Part and Map Preloading could reduce LL-HLS latency by an additional part duration. This feature was left out of v1.0 because of timing and the complexity involved in dropping hinted parts in situations that still need to be cataloged and tested. We also lacked multiple MAP support which has not landed in v1.0.4.

Feature summary and resources:

1. Parse EXT-X-PRELOAD-HINT tags and attributes:
   • #EXT-X-PRELOAD-HINT:TYPE=MAP,URI=”init.mp4”preload init segments (usually followed by a discontinuity)
   • #EXT-X-PRELOAD-HINT:TYPE=PART,URI=”part.m4s”
   • with byterange (length known)
     #EXT-X-PRELOAD-HINT:TYPE=PART,URI=”part.m4s”,BYTERANGE-START=0,BYTERANGE-LENGTH=4044
   • with byterange chunk-transfer
     #EXT-X-PRELOAD-HINT:TYPE=PART,URI=”part.m4s”,BYTERANGE-START=0
2. Make preload-hint requests and handle the response
   • request part files and hold result for part update (no byterange)
   • request part byterange and hold result for part update (length known)
   • stream part file, hold result for part update byte-range(s) is received in part-update (chunk-transfer)
   • do not sample TTFB for blocked part-hint responses
3. Handle part updates following preload-hint
   a. If part update matches preload-hint request/response push update
   b. If part update invalidates preload-hint drop request/response

https://developer.apple.com/wwdc20/10229
https://developer.apple.com/wwdc20/10232