chore(pubsub): migrate to std::optional

[colinmoy]

Replacing explicit usages of absl::optional with std::optional in the PubSub Client library as part of broader modernization efforts

[gemini-code-assist]

Code Review

This pull request migrates the codebase from absl::optional to std::optional (along with std::nullopt and std::make_optional) across various files in the google/cloud/storage and google/cloud/pubsub libraries, including tests, benchmarks, and examples. It also removes the absl::optional dependency from the CMake configuration. There are no review comments, and the changes align with modern C++17 standards as outlined in the repository style guide. I have no additional feedback to provide.

[codecov]

Codecov Report

❌ Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.27%. Comparing base (40d3ab0) to head (5d7c4a3).

Files with missing lines	Patch %	Lines
...cloud/pubsub/internal/subscription_session_test.cc	71.42%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16231   +/-   ##
=======================================
  Coverage   92.27%   92.27%           
=======================================
  Files        2214     2214           
  Lines      206505   206504    -1     
=======================================
+ Hits       190549   190552    +3     
+ Misses      15956    15952    -4     

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