chore: migrate common libraries to std::optional

[colinmoy]

Replacing explicit usages of absl::optional with std::optional in the Common libraries and internal utilities (google/cloud/ and google/cloud/internal/) as part of broader modernization efforts

[gemini-code-assist]

Code Review

This pull request replaces the use of absl::optional, absl::nullopt, and absl::make_optional with their standard C++17 equivalents, std::optional, std::nullopt, and std::make_optional, across various files in the repository. It also updates the relevant #include directives and build dependencies accordingly. There are no review comments, and I have no feedback to provide.

[codecov]

Codecov Report

❌ Patch coverage is 96.34703% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.29%. Comparing base (93c8274) to head (c926366).

Files with missing lines	Patch %	Lines
google/cloud/internal/curl_handle_factory.h	0.00%	4 Missing ⚠️
google/cloud/credentials.cc	50.00%	1 Missing ⚠️
...e/cloud/internal/async_read_write_stream_timeout.h	80.00%	1 Missing ⚠️
.../cloud/internal/async_streaming_read_rpc_timeout.h	80.00%	1 Missing ⚠️
...internal/openssl/parse_service_account_p12_file.cc	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16254   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files        2221     2221           
  Lines      207359   207359           
=======================================
+ Hits       191372   191373    +1     
+ Misses      15987    15986    -1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.