impl(bigtable): add option to select directpath initialization mode

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces DirectPathInitializationModeOption to allow configuring the initialization behavior of DirectPath in Google Cloud Bigtable, supporting both blocking and speculative asynchronous modes. The feedback suggests refactoring MakeBlockingDirectPathDataConnection to eliminate duplicated StubManager creation logic using a helper lambda, and optimizing MakeDirectPathDataConnection to avoid unnecessary copies of components.options while preventing potential order-of-evaluation issues.

[codecov]

Codecov Report

❌ Patch coverage is 73.45133% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.26%. Comparing base (17ad4f5) to head (8b3a906).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...d/bigtable/tests/observability_integration_test.cc	42.42%	19 Missing ⚠️
google/cloud/bigtable/data_connection.cc	81.03%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16391      +/-   ##
==========================================
+ Coverage   92.24%   92.26%   +0.01%     
==========================================
  Files        2246     2246              
  Lines      212035   212121      +86     
==========================================
+ Hits       195598   195710     +112     
+ Misses      16437    16411      -26     

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