[Drain] Support extensible element metadata propagation in ReduceFnRunner

[stankiewicz]

Description

This PR refactors the metadata propagation logic in ReduceFnRunner to support extensible metadata.

Previously, metadata tracking (specifically causedByDrain) was not stored in state at all during execution in ReduceFnRunner, which caused metatada loss failures when firing timers or merging panes. This PR fixes that by introducing a unified state map for element metadata.

To make it easier to add future payloads (such as OpenTelemetry context maps or CDC insert/update markers) without modifying method signatures, this change groups targeted fields into a unified container and offloads combination rules to a clean aggregator class.

Key Changes

• Runner Fix: Added stateful tracking using METADATA_TAG (persisting CombinedMetadata) in ReduceFnRunner, ensuring metadata is no longer lost during grouping.
• Extensible Container: Created CombinedMetadata (guided by @AutoValue) that groups element metadata together.
• Metadata Combiners: Encapsulated combination logic within CombinedMetadataCombiner.

Open question

• is it breaking change?

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[stankiewicz]

R: @kennknowles

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