ci: code updates to get windows builds working

[scotthart]

This PR makes the necessary code updates to allow MSVC to build the SDK. Several issues were created to track tests that were disabled in windows. Another issue was created to determine if GDCH credentials support is required on Windows.

[gemini-code-assist]

Code Review

This pull request disables several tests on Windows/MSVC due to compatibility issues, updates HTTP header string construction to use absl::StrCat, and modifies a lambda capture in oauth2_regional_access_boundary_token_manager.h. The review feedback highlights a critical issue with the lambda capture: using default capture by value [=] implicitly captures this in an asynchronous context, which can lead to use-after-free bugs. It is recommended to explicitly capture only the required variables and use std::move to avoid unnecessary copies of the request object.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.22%. Comparing base (a82ff3b) to head (9f1dcc2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16178      +/-   ##
==========================================
- Coverage   92.23%   92.22%   -0.01%     
==========================================
  Files        2265     2265              
  Lines      209832   209832              
==========================================
- Hits       193534   193527       -7     
- Misses      16298    16305       +7     

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