feat: spannertest-changestreams

[christogav]

Initial implementation thoughts for implementing Change Streams in spannertest. I've tried to implement key features to support in-memory testing of change streams.

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[gemini-code-assist]

Code Review

This pull request implements change stream support within the spannertest emulator, enabling the creation, alteration, and deletion of change streams via DDL. It introduces mutation logging for all write operations (Insert, Update, Delete, Replace) and implements the READ_ table-valued function (TVF) to allow users to query change records. The changes also include updates to the spansql parser to handle TVF syntax and provide exported Go types for client-side decoding. Feedback identifies a critical bug where unconditional transaction lookups in ExecuteSql would break standard queries, and a logic error in the change stream iterator that could incorrectly merge records from distinct transactions sharing the same commit timestamp.