Adds some extra logging to GCP Spanner Changestream Source

[acrites]

Adds logging of creation as well as min watermark to changestream source to help debug potential watermark moving backwards.

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[gemini-code-assist]

Summary of Changes

Hello @acrites, 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 significantly advances Apache Beam's stateful processing by fully enabling and implementing MultimapState support within the Fn API, allowing pipelines to efficiently manage and retrieve all entries from multimap user states. Alongside this major functional addition, a minor but important logging improvement has been integrated into the GCP Spanner Changestream Source to provide more detailed information for debugging potential watermark regressions.

Highlights

• MultimapState Fn API Support: The pull request introduces full implementation for MultimapState within the Beam Fn API, enabling users to retrieve all entries from a multimap state. This involves adding a new MultimapEntriesUserState message in the beam_fn_api.proto and providing a concrete implementation for the bindMultimap method in FnApiStateAccessor.
• MultimapState Entry Retrieval: The MultimapUserState class now supports fetching all key-value entries, enhancing the capabilities of stateful processing. Corresponding tests have been added to validate this new functionality.
• Typo Correction: A consistent typo from runnerCapabilites to runnerCapabilities has been corrected across the Fn API state accessor, improving code consistency and readability.
• Spanner Changestream Logging Enhancement: Logging for the GCP Spanner Changestream Source has been enhanced to include partition creation time and output timestamp. This additional detail will aid in debugging potential watermark issues.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.80%. Comparing base (d26dbac) to head (60c258e).
⚠️ Report is 57 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #36289      +/-   ##
============================================
+ Coverage     54.93%   56.80%   +1.87%     
- Complexity     1617     3387    +1770     
============================================
  Files          1057     1220     +163     
  Lines        164425   185691   +21266     
  Branches       1165     3523    +2358     
============================================
+ Hits          90320   105476   +15156     
- Misses        71955    76881    +4926     
- Partials       2150     3334    +1184     

Flag	Coverage Δ
java	70.41% <100.00%> (+2.04%)	⬆️
python	80.91% <ø> (+<0.01%)	⬆️

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