feat(spanner): add support for Queues in mutations

[scotthart]

Adds C++ client library support for Cloud Spanner Queues (Mutation::Send and Mutation::Ack):

• Introduces spanner::SendMutationBuilder and spanner::MakeSendMutation (supporting optional deliver_time) to enqueue messages into a Spanner queue.
• Introduces spanner::AckMutationBuilder and spanner::MakeAckMutation (supporting optional ignore_not_found) to acknowledge messages in a Spanner queue.
• Adds unit tests in mutations_test.cc and end-to-end Enterprise edition integration tests in queue_integration_test.cc.

[gemini-code-assist]

Code Review

This pull request adds support for Spanner Queues by introducing SendMutationBuilder, AckMutationBuilder, and helper functions, along with unit and integration tests. The review feedback points out a potential undefined behavior issue in mutations.cc where a StatusOr is dereferenced without checking .ok(), recommending the use of .value() instead. In queue_integration_test.cc, the feedback suggests replacing raw pointers with std::optional for safer lifetime management, and replacing several instances of auto in loops with explicit types to comply with the style guide regarding obscured StatusOr types.

[codecov]

Codecov Report

❌ Patch coverage is 45.18519% with 148 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.33%. Comparing base (d1a2175) to head (91ea6c4).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...panner/integration_tests/queue_integration_test.cc	5.12%	148 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16549      +/-   ##
==========================================
- Coverage   92.39%   92.33%   -0.07%     
==========================================
  Files        2262     2263       +1     
  Lines      218382   218652     +270     
==========================================
+ Hits       201775   201893     +118     
- Misses      16607    16759     +152     

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