[Dataflow Streaming] Add WindmillTagEncodingV2.

[arunpandianp]

This adds a new way of encoding windmill state tags and timer tags.
The new code is behind an unstable experiment. More work is needed
before it can be used on real workloads.

This is on top of #37150.

[arunpandianp]

R: @scwhittle

[gemini-code-assist]

Summary of Changes

Hello @arunpandianp, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant architectural change to how state and timer tags are encoded within the Windmill backend for Dataflow streaming jobs. By abstracting the encoding logic behind a new WindmillTagEncoding interface, it paves the way for WindmillTagEncodingV2, an experimental encoding scheme. This refactoring allows for future improvements in efficiency or functionality of tag handling, with the new version being opt-in via an experiment flag for cautious evaluation.

Highlights

• New Windmill Tag Encoding (V2): Introduced WindmillTagEncodingV2, a new way of encoding Windmill state tags and timer tags. This new encoding is currently behind an unstable experiment flag and is not yet ready for production workloads.
• Refactoring with WindmillTagEncoding Interface: A new abstract class WindmillTagEncoding has been added, defining a common interface for encoding state and timer tags. The existing WindmillStateTagUtil has been renamed to WindmillTagEncodingV1 and now implements this interface, maintaining backward compatibility.
• Dynamic Encoding Selection: The StreamingModeExecutionContext now dynamically selects between WindmillTagEncodingV1 and WindmillTagEncodingV2 based on an experiment flag, allowing for controlled rollout and testing of the new encoding.
• Core State API Enhancements: Added getWindowCoder() methods to StateNamespaces classes and getKind() and isSystemTagInternal() methods to StateTags to support the new encoding logic.
• Widespread Integration: Multiple components, including WindmillKeyedWorkItem, WindmillTimerInternals, CachingStateTable, WindmillCombiningState, and WindmillStateInternals, have been updated to use the new WindmillTagEncoding interface, ensuring consistent tag handling across the Dataflow worker.

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[github-actions]

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[scwhittle]

Just some initial comments, might be able to merge/rebase since the base refactoring is merged. I'll be OOO for two weeks