chore: migrate from deprecated absl types to std equivalents

[scotthart]

There are no logic changes in this PR, simply updating to C++17 std types that used to use absl types. Changes in the use of auto will be made in a separate PR.

fixes #16354

[gemini-code-assist]

Code Review

This pull request migrates various Abseil utilities (such as absl::variant, absl::visit, absl::holds_alternative, absl::get, absl::conjunction, absl::disjunction, and absl::void_t) to their standard C++17 library equivalents (std::variant, std::visit, etc.) across the codebase. The review feedback identifies a potential compilation failure in default_row_reader.cc due to mismatched return types, an unused Abseil header in list_objects_and_prefixes_reader.h, and a missed migration from absl::optional to std::optional in validate_metadata.cc.

[codecov]

Codecov Report

❌ Patch coverage is 87.42138% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.26%. Comparing base (f69f92f) to head (89b448d).

Files with missing lines	Patch %	Lines
...igquery/v2/minimal/internal/common_v2_resources.cc	60.00%	2 Missing ⚠️
google/cloud/internal/future_then_impl.h	50.00%	2 Missing ⚠️
...torage/internal/async/writer_connection_resumed.cc	83.33%	2 Missing ⚠️
google/cloud/testing_util/is_proto_equal.cc	66.66%	2 Missing ⚠️
..._tests/golden/v1/golden_kitchen_sink_connection.cc	0.00%	1 Missing ⚠️
generator/internal/http_option_utils.cc	80.00%	1 Missing ⚠️
generator/internal/longrunning.cc	91.66%	1 Missing ⚠️
generator/internal/metadata_decorator_generator.cc	0.00%	1 Missing ⚠️
...ator/internal/metadata_decorator_rest_generator.cc	0.00%	1 Missing ⚠️
...bigquery/v2/minimal/internal/dataset_connection.cc	0.00%	1 Missing ⚠️
... and 6 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16400      +/-   ##
==========================================
+ Coverage   92.24%   92.26%   +0.01%     
==========================================
  Files        2246     2246              
  Lines      212136   212135       -1     
==========================================
+ Hits       195692   195719      +27     
+ Misses      16444    16416      -28     

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[scotthart]

/gemini review

[gemini-code-assist]

Code Review

This pull request migrates the codebase from Abseil's variant, optional, and related utilities to their standard C++17 library equivalents, updating both source files and build configurations (Bazel and CMake). The review feedback focuses on enforcing the repository's style guide regarding type deduction (auto). Specifically, it highlights multiple instances where auto is used inappropriately, such as when it obscures domain objects (like Status, ReadPayload, and ObjectMetadata), hides primitive/scalar types, or leads to unnecessary copies instead of using const&.