Track.extras (userData) for Playlist

Current implementation is trying to add ExtrasNamespace to track object rather than Track.extras

Description

This change sets the ExtrasNamespace correctly to Track.extras

[x] If code changes were made then they have been tested.
- [x] I have updated the documentation to reflect the changes.
- [ ] I have updated the changelog with a quick recap of my changes.
[x] This PR fixes an issue.
[ ] This PR adds something new (e.g. new method or parameters).
[ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
[ ] This PR is not a code change (e.g. documentation, README, ...)
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