impl(spanner): add request ID generation functions

[scotthart]

This PR adds inert code (to be used later) to generate and format Request ID headers for Spanner. Per the process specific requirements, there is some additional logic on non-Windows platforms to handle forking of processes.

[gemini-code-assist]

Code Review

This pull request introduces a new utility for generating and formatting Spanner request IDs, including its implementation, unit tests, and build configuration updates. The review feedback recommends addressing a potential fork deadlock risk and performance bottleneck in ProcessRandomId by replacing the std::mutex with a lock-free atomic pattern. Additionally, it suggests capturing and verifying the return value of the write system call in the unit tests to prevent compiler warnings under strict build settings.

[dbolduc]

nit: why not check for Eq(1), Eq(2), Eq(3) ?

and monotonic would be more relaxed like:

  EXPECT_THAT(c2, Gt(c1));
  EXPECT_THAT(c3, Gt(c2));

[codecov]

Codecov Report

❌ Patch coverage is 87.01299% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.34%. Comparing base (a892ceb) to head (bca26da).

Files with missing lines	Patch %	Lines
.../cloud/spanner/internal/spanner_request_id_test.cc	83.33%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16465      +/-   ##
==========================================
+ Coverage   92.31%   92.34%   +0.02%     
==========================================
  Files        2246     2248       +2     
  Lines      214568   214645      +77     
==========================================
+ Hits       198086   198220     +134     
+ Misses      16482    16425      -57     

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