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 93.10345% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.24%. Comparing base (a9075b2) to head (77d0ae7).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
...cloud/pubsub/internal/subscription_session_test.cc	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16231      +/-   ##
==========================================
- Coverage   92.24%   92.24%   -0.01%     
==========================================
  Files        2265     2265              
  Lines      210126   210126              
==========================================
- Hits       193833   193823      -10     
- Misses      16293    16303      +10     

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