chore(storage): migrate to std::optional

[colinmoy]

Replacing explicit usages of absl::optional with std::optional in the Storage 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 across various files in the Google Cloud Storage library, benchmarks, and tests. The review identified two critical issues: a missing # symbol in the #include <optional> directive in bucket_cors_entry.h which will cause a compilation error, and an incorrect attempt to link std::optional as a CMake target in google_cloud_cpp_storage_grpc.cmake instead of simply removing the absl::optional dependency.

[gemini-code-assist]

The # symbol is missing from the #include <optional> directive, which will cause a compilation error.

#include <optional>

[gemini-code-assist]

std::optional is a C++ standard library feature and not a CMake target. It should not be listed in target_link_libraries. Since the codebase is migrating to std::optional, the absl::optional dependency should simply be removed.

           gRPC::grpc++

[codecov]

Codecov Report

❌ Patch coverage is 92.04545% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.27%. Comparing base (40d3ab0) to head (a6cac0c).

Files with missing lines	Patch %	Lines
google/cloud/storage/benchmarks/benchmark_utils.cc	0.00%	10 Missing ⚠️
...le/cloud/storage/internal/async/read_range_test.cc	0.00%	2 Missing ⚠️
...orage/internal/async/writer_connection_buffered.cc	0.00%	2 Missing ⚠️
...rage/internal/async/writer_connection_impl_test.cc	91.66%	2 Missing ⚠️
...ud/storage/internal/object_write_streambuf_test.cc	90.90%	2 Missing ⚠️
google/cloud/storage/async/writer_connection.h	0.00%	1 Missing ⚠️
google/cloud/storage/benchmarks/bounded_queue.h	0.00%	1 Missing ⚠️
...rnal/async/object_descriptor_connection_tracing.cc	0.00%	1 Missing ⚠️
google/cloud/storage/internal/async/read_range.cc	66.66%	1 Missing ⚠️
...rage/internal/async/writer_connection_finalized.cc	0.00%	1 Missing ⚠️
... and 5 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16229      +/-   ##
==========================================
- Coverage   92.27%   92.27%   -0.01%     
==========================================
  Files        2214     2214              
  Lines      206505   206504       -1     
==========================================
- Hits       190549   190546       -3     
- Misses      15956    15958       +2     

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