Add track processor API (#711)
* model insertable streams api
* kitten processor, virtual background api/processor
* init support for stream processors
* processor stability updates
* simplify processor handling
* add notes
* fix types
* Update src/room/track/LocalTrack.ts
Co-authored-by: David Zhao <dz@livekit.io>
* remove console log
* Create lovely-trains-visit.md
* change changeset to minor
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Co-authored-by: David Zhao <dz@livekit.io>
