fix: Linear DASH multiperiod label issue

[adrums86]

Description

Fixes Linear DASH SSAI Label Issue
Linear multi-period DASH with <Label> elements that roll in and out of continuous AdaptationSets cause the stream to stall because we use the value of the <Label> as the key for the mediaGroups.type.group.<labelKey> object and the associated mediaGroup group/playlist ID. When the manifest updates and reaches the period either with a <Label> or omitting the <Label> we can no longer access that media group or playlists to update the playlist as we rely on the labelKey set from the previous manifest/period to look for new segments in the update.

Specific Changes proposed

• Change the way we create groupIDs for mediaGroup playlists to allow for DASH playlists to use the representation ID rather than the label from the parser. Per DASH IF, continuous media in separate periods share the same representation IDs across period boundaries, so this change can easily utilize that feature to ensure our playlistIDs are consistent between continuous periods.
• Add the new labelKey to the mediaGroups.type.group object and add the playlist reference.
• Remove any old labelKeys from the mediaGroups.type.group object that no longer exist in the new manifest update.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@afb1ff7). Click here to learn what that means.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1352   +/-   ##
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  Files           ?       40           
  Lines           ?     9842           
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  Hits            ?     8466           
  Misses          ?     1376           
  Partials        ?        0           

Impacted Files	Coverage Δ
src/dash-playlist-loader.js	90.33% <100.00%> (ø)
src/manifest.js	93.57% <100.00%> (ø)

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[alex-barstow]

Should we change _ to properties to be consistent with other forEachMediaGroup() calls in this file?

[adrums86]

Good call, I have an old lint-driven habit of using _ for any unused parameters. Consistency > arbitrary lint rules IMO.

[wseymour15]

Looks good! Just have that one optional change.

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