• Adds tracking for keys used within a window, so the OnWindowExpiration callback can be invoked.
• Restricts the output watermark from advancing until a window's been garbage collected and all keys have been expired.
• Added a "side channel" mechanism to insert bundles for execution outside of normal execution flow.
  • This may ultimately be used for ProcessingTime timers, as well as for Triggers/
• Supports OnWindowExpiration, enabling initial GroupIntoBatches support.

Fixes #32211

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