impl(bigtable): add Directpath prober

[scotthart]

Probes to see if a directpath connection is possible.

[gemini-code-assist]

Code Review

This pull request introduces a new DirectPathProber utility class and its corresponding unit tests to probe DirectPath startup compatibility, along with a new DirectPathProbeTimeoutOption configuration option. Feedback on the changes highlights a potential RPC failure if the required PingAndWarmRequest::name field is left empty, suggesting a fallback mechanism using project and instance ID options. Additionally, a style guide violation was noted regarding the use of absl::string_view instead of std::string_view.

[codecov]

Codecov Report

❌ Patch coverage is 97.27047% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.27%. Comparing base (d2bb2bb) to head (57e1541).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
.../cloud/bigtable/internal/directpath_prober_test.cc	96.45%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16376      +/-   ##
==========================================
- Coverage   92.27%   92.27%   -0.01%     
==========================================
  Files        2239     2241       +2     
  Lines      210816   211219     +403     
==========================================
+ Hits       194537   194905     +368     
- Misses      16279    16314      +35     

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[colinmoy]

There seems to be missing test coverage for IsDirectPathIp and ExtractIpPreference