Provide a robust Go SDK Spanner IO for Change Streams. See CHANGESTRE…

[christogav]

Provide a robust Go SDK Spanner IO for Change Streams. See CHANGESTREAM.md for more detailed information on the approach taken.

Note to the reviewer: I've been considering this for some time as we have had a desire to not be tied to the Java implementation. We may want to discuss approach and any concerns before deep diving the implementation itself.

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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 adds a robust Go SDK implementation for reading Spanner Change Streams. By leveraging Splittable DoFns, the connector provides a scalable, dependency-free approach to change data capture that integrates directly with Beam's native checkpointing for durability. This implementation aims to remove the dependency on Java-based metadata tables, offering a more streamlined experience for Go-based pipelines.

Highlights

• New Spanner Change Stream Connector: Introduced a native Go implementation of the Spanner Change Stream connector using a Splittable DoFn (SDF) to handle partition discovery and reading.
• Architecture and Scalability: The connector utilizes aggressive TrySplit to achieve per-partition parallelism without requiring an external metadata database, relying instead on the Beam runner's checkpointing mechanism.
• Resilience and Watermarking: Implemented robust handling for transient gRPC errors (e.g., UNAVAILABLE, ABORTED) via checkpoint-and-retry, and added a watermark estimator to ensure correctness during partition splitting.

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