WIP Add custom window support in Go SDK

[hnnsgstfssn]

Implement custom WindowFn registration for the Go SDK, reaching capability parity with Java/Python AssignContext.

Custom WindowFns are plain structs registered via RegisterWindowFn and validated structurally at registration time, matching the DoFn pattern used elsewhere in the SDK. Two AssignWindows shapes are accepted:

AssignWindows(typex.EventTime) []typex.Window
AssignWindows(typex.EventTime, T) []typex.Window

The second form is element-aware: the element value drives which window(s) it lands in. Validation happens via reflection at registration, so misuse fails at pipeline construction rather than at runtime. A package-level registry records the struct type and optional element type for cross-package lookup via LookupWindowFnMeta.

An interface-based shape (WindowAssigner) was explored first. Structural typing was chosen instead because it keeps custom WindowFns consistent with DoFns, avoids forcing users to satisfy a Go-specific interface, and lets the same registry carry the element-type metadata that the dispatch and translation paths need.

WindowFnInvoker dispatches in three tiers: typed interface (zero alloc), element-aware typed interface (zero alloc), and reflect.Value.Method.Call as a fallback (2 allocs/element).

Serialization piggybacks on the Beam model proto (FunctionSpec with URN + JSON payload), so the internal v1 proto needs no new fields. The side-input mapping path panics for element-aware WindowFns, matching the existing sessions guard, since side-input windows have no element to feed AssignWindows.

Integration coverage exercises an element-aware WindowFn end-to-end through the direct runner, verifying that elements with different SizeMs values land in the windows dictated by their own data.

Adresses #20627

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Summary of Changes

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This pull request introduces support for custom WindowFns in the Go SDK, achieving capability parity with the Java and Python SDKs. By adopting a structural typing approach consistent with the existing DoFn pattern, it allows users to define custom windowing logic while ensuring type safety and performance through an optimized invocation layer. The changes include a new registration system, serialization support, and robust integration testing to verify correct behavior across different windowing scenarios.

Highlights

• Custom WindowFn Registration: Implemented a registration mechanism for custom WindowFns using a struct-based pattern, allowing for structural validation at registration time.
• Element-Aware Windowing: Added support for element-aware WindowFns, enabling window assignment logic to depend on the element value itself.
• Dispatch Optimization: Introduced a WindowFnInvoker that provides optimized dispatch paths (typed interface vs. reflection) to minimize allocation overhead during window assignment.
• Serialization and Integration: Integrated custom WindowFns with the existing Beam model proto serialization and added comprehensive integration tests covering both standard and element-aware custom windowing.

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