build: add benchmark build scaffolding

[timothyw553]

Summary

Adds an opt-in benchmark build path so future C++ microbenchmarks can live in-tree without affecting normal builds. Benchmarks stay disabled by default, and this PR keeps the executable intentionally small so reviewers can focus on CMake/Meson wiring and Google Benchmark integration.

• Adds ICEBERG_BUILD_BENCHMARKS for CMake and a disabled-by-default benchmarks Meson feature.
• Wires src/iceberg/benchmark into both build systems.
• Adds Google Benchmark dependency setup for CMake and Meson.
• Adds a tiny smoke benchmark so the new target is buildable in isolation.

Stack

This is PR 1 of 2.

PR 2 adds the actual metrics evaluator benchmark cases on top of this scaffolding: timothyw553#1.

Test Plan

• Configured benchmark-enabled CMake build with -DICEBERG_BUILD_BENCHMARKS=ON.
• Built metrics_evaluator_benchmark on this branch.
• Ran the smoke benchmark with --benchmark_min_time=0.01s.
• Re-ran the default build and test suite on the top of the stack: 18/18 tests passed.
• Meson wiring is syntax-reviewed but not locally executed because meson is not installed in this shell.

Verification Commands

cmake -S . -B build-bench-stack -G Ninja \
  -DFETCHCONTENT_TRY_FIND_PACKAGE_MODE=NEVER \
  -DCMAKE_IGNORE_PREFIX_PATH=/opt/homebrew/anaconda3 \
  -DCMAKE_PREFIX_PATH='/opt/homebrew/opt/snappy;/opt/homebrew/opt/zstd' \
  -DICEBERG_BUILD_BENCHMARKS=ON
cmake --build build-bench-stack --target metrics_evaluator_benchmark
build-bench-stack/src/iceberg/benchmark/metrics_evaluator_benchmark --benchmark_min_time=0.01s
cmake --build build
ctest --test-dir build --output-on-failure

[zhjwpku]

nit: should we use the commit SHA like gtest?

[timothyw553]

Done. I pinned Google Benchmark to the v1.8.4 release commit SHA (a4cf155615c63e019ae549e31703bf367df5b471) and kept the version as an inline comment, matching the gtest style.