impl(bigtable): add DirectPath diagnostics

[scotthart]

Adds diagnostics check for determining DirectPath status.

[gemini-code-assist]

Code Review

This pull request introduces DirectPath diagnostics for Google Cloud Bigtable, adding the DirectPathDiagnostics class, associated network system interfaces, and unit tests. Key feedback highlights a critical bug where inet_pton failures could cause false positives by connecting to localhost, and another issue where the calculated timeout in RunAsync is unused and not captured by the lambda. Additionally, recommendations were made to optimize interface name comparisons using std::strcmp, pass std::shared_ptr arguments by const& to avoid atomic reference count overhead, and remove default parameters in the internal namespace to comply with the repository style guide.

[codecov]

Codecov Report

❌ Patch coverage is 96.84466% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.29%. Comparing base (f44759f) to head (db661d2).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...d/bigtable/internal/directpath_diagnostics_test.cc	97.59%	7 Missing ⚠️
.../cloud/bigtable/internal/directpath_diagnostics.cc	95.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16378      +/-   ##
==========================================
+ Coverage   92.27%   92.29%   +0.02%     
==========================================
  Files        2241     2244       +3     
  Lines      211219   211631     +412     
==========================================
+ Hits       194904   195329     +425     
+ Misses      16315    16302      -13     

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