DateRange parsing and validation enhancements for Interstitials

[robwalch]

This PR will...

• Add DateRange parsing and validation enhancements for Interstitials:
  • Parse CUE attribute (and legacy X-CUE)
  • Anchor DateRange time to nearest EXTINF (tagAnchor: Fragment) and DISCONTINUITY domain. This keeps DateRange object start times aligned to the media buffer as parsed times diverge from playlist times. (Does not adjust TextTrack cues: #6203).
  • Maintain Playlist DateRange index (tagOrder) for ordering of Preroll and Postrolls
• Warn when attributes expected to have quoted-attribute values are missing quotes (Relates to #6203)
• Move Variable Substitution logic into AttrList where all quoted-string attribute values can have variable definition replacement performed
• Add playlistOffset to Fragment objects so that a reference to the original Playlist start offset of each segment is available even after Fragment start is adjusted to match startPTS
• Adjust DateRange metadata Cue start and end times when Playlist segment start times differ from media segment start times by more than 1/100th of a second 0c5ca21 (Resolves #6203)

Why is this Pull Request needed?

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

Resolves issues:

• Resolves #6203
• Related to #5730

Checklist

• [x] changes have been done against master branch, and PR does not conflict
• [x] new unit / functional tests have been added (whenever applicable)
• [x] API or design changes are documented in API.md