Add ValueKind element metadata

[ahmedabu98]

Adds ValueKind to the model.

Allows DoFns to output elements with a ValueKind, and receive elements with a ValueKind parameter (in the @ProcessElement method)

Support enabled in DirectRunner
Touches parts of the Dataflow legacy worker, but further work would need to be done internally

Added corresponding logic to FnAPI, but Portable runners (like Dataflow V2) would need to extend it and enable on their side

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Summary of Changes

Hello, 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 support for Change Data Capture (CDC) metadata by adding a ValueKind concept throughout the Apache Beam SDK and Fn API. This allows for explicit tracking of data changes (INSERT, UPDATE, DELETE) in windowed elements, ensuring this metadata is correctly propagated through the execution pipeline, including serialization in Windmill and accessibility within user-defined DoFn logic.

Highlights

• ValueKind Introduction: Introduced a new ValueKind enum to represent CDC metadata (INSERT, UPDATE_BEFORE, UPDATE_AFTER, DELETE) and integrated it into the Beam Fn API and core SDK.
• API Expansion: Added outputWithKind methods to DoFn.WindowedContext and related runners to allow users to specify the ValueKind during output operations.
• Dataflow Integration: Updated WindmillSink and UngroupedWindmillReader to correctly encode and decode ValueKind metadata in Windmill messages.
• Reflect and Invoker Updates: Updated DoFnInvoker and ByteBuddyDoFnInvokerFactory to support ValueKind as a parameter in DoFn methods.

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[ahmedabu98]

Added this just to visualize. It's likely more correct to let each runner set it on their side