impl(v3): add generator support for connection cq access

[scotthart]

This is necessary to implement bigtable::WaitForConsistency using the generated admin types.

[gemini-code-assist]

Summary of Changes

Hello @scotthart, 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 enhances the code generation framework by adding a new configuration option that allows specific service connections to expose their internal CompletionQueue. This feature is crucial for scenarios like bigtable::WaitForConsistency, which requires direct access to the underlying CompletionQueue for asynchronous operations. The changes ensure that generated client libraries can provide this access when configured, without exposing it by default.

Highlights

• New Generator Option: Introduced emit_completion_queue_accessor in generator_config.proto to control the generation of a CompletionQueue accessor for service connections.
• Bigtable Admin Integration: Enabled this new option for the BigtableTableAdmin service in generator_config.textproto, specifically to support bigtable::WaitForConsistency.
• Code Generation Logic: Updated the code generator to conditionally include completion_queue.h and generate a protected virtual completion_queue() method in the Connection interface, along with a friend function in the internal namespace.
• Implementation Generation: The ConnectionImpl generator now produces an override for the completion_queue() method, returning the CompletionQueue from the background threads.
• Bigtable Admin Connection Update: The generated BigtableTableAdminConnection and BigtableTableAdminConnectionImpl classes now include the necessary CompletionQueue accessor.

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[gemini-code-assist]

Code Review

This pull request introduces a new configuration option, emit_completion_queue_accessor, to the service configuration. This option enables the generation of a protected accessor and a friend function to expose the CompletionQueue from the BackgroundThreads owned by the Connection class. This functionality is specifically noted as necessary for bigtable::WaitForConsistency. The changes are consistently applied across the generator's logic and the generated Bigtable Table Admin client code. The implementation appears correct and adheres to the repository's style guide and best practices. No critical, high, or medium severity issues were found.

[codecov]

Codecov Report

❌ Patch coverage is 48.00000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.87%. Comparing base (f944a14) to head (c98296c).
⚠️ Report is 1 commits behind head on prepare-for-v3.0.0.

Files with missing lines	Patch %	Lines
generator/internal/connection_generator.cc	37.50%	10 Missing ⚠️
generator/internal/connection_impl_generator.cc	50.00%	3 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           prepare-for-v3.0.0   #15912   +/-   ##
===================================================
  Coverage               92.87%   92.87%           
===================================================
  Files                    2343     2343           
  Lines                  216117   216141   +24     
===================================================
+ Hits                   200710   200741   +31     
+ Misses                  15407    15400    -7     

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