Add missing multivariant playlist tags and attributes from draft 15

[theRealRobG]

Description

This PR brings section 4.4.6 Multivariant Playlist Tags up to date with draft 15 of the specification.

This means a few new tags are added, and there are a lot of updated attributes too, so the various validators have been updated to reflect this.

I intend to bring sections 4.4.2, 4.4.3, 4.4.4, and 4.4.5 up to date in separate PRs.

Change Notes

• Added new tags:
  • EXT-X-SESSION-DATA
  • EXT-X-SESSION-KEY
  • EXT-X-CONTENT-STEERING
• Added attributes for EXT-X-MEDIA:
  • STABLE-RENDITION-ID
  • BIT-DEPTH
  • SAMPLE-RATE
  • CHANNELS
• Added attributes for EXT-X-STREAM-INF and EXT-X-I-FRAME-STREAM-INF:
  • AVERAGE-BANDWIDTH
  • SCORE
  • SUPPLEMENTAL-CODECS
  • FRAME-RATE
  • HDCP-LEVEL
  • ALLOWED-CPC
  • VIDEO-RANGE
  • REQ-VIDEO-LAYOUT
  • STABLE-VARIANT-ID
  • PATHWAY-ID
• Updated existing validators and created new validators to account for additional and updated tags.

Pre-submission Checklist

• I ran the unit tests locally before checking in.
• I made sure there were no compiler warnings before checking in.
• I have written useful documentation for all public code.
• I have written unit tests for this new feature.

[rmigneco]

Thanks for addressing comments