impl(bigtable): add snippet demonstrating query execution

[scotthart]

No description provided.

[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 Bigtable C++ client library examples by introducing a comprehensive demonstration of SQL query execution. It refactors the example command handling to better support client-centric operations and integrates the new query example into the automated example suite and as a standalone command-line option, improving clarity and usability for developers learning to interact with Bigtable via SQL.

Highlights

• New Query Execution Example: A new example, PrepareAndExecuteQuery, has been added to demonstrate how to prepare and execute SQL queries against Bigtable using the C++ client library, including parameter binding and result processing.
• Example Infrastructure Enhancement: The example command infrastructure has been extended with a new MakeCommandEntry overload and ClientCommandType to simplify the creation of example commands that operate directly on a google::cloud::bigtable::Client object.
• Doxygen Snippet Integration: The google/cloud/bigtable/client.h header now uses a Doxygen @snippet directive to reference the new query execution example, keeping the header documentation concise and up-to-date with external examples.

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[gemini-code-assist]

Code Review

This pull request adds a new code snippet demonstrating how to execute a query with the Bigtable C++ client. The changes are generally good, but I've found a couple of issues that prevent the new example from running correctly. One is a critical bug where the example code is not actually called, and another is a misconfiguration in the command-line argument parsing. Please see my detailed comments.

[codecov]

Codecov Report

❌ Patch coverage is 34.14634% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.94%. Comparing base (6d79425) to head (05dac5f).

Files with missing lines	Patch %	Lines
google/cloud/bigtable/examples/data_snippets.cc	7.69%	24 Missing ⚠️
...loud/bigtable/examples/bigtable_examples_common.cc	80.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15753      +/-   ##
==========================================
- Coverage   92.95%   92.94%   -0.01%     
==========================================
  Files        2448     2448              
  Lines      226391   226434      +43     
==========================================
+ Hits       210442   210464      +22     
- Misses      15949    15970      +21     

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