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 92.27%. Comparing base (d26d929) to head (199ab4b).

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   92.27%   92.27%   -0.01%     
==========================================
  Files        2217     2217              
  Lines      206628   206628              
==========================================
- Hits       190672   190669       -3     
- Misses      15956    15959       +3     

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