GH-46207: [C++] Rename arrow::util::StringBuilder and move to internal namespace

[Ziy1-Tan]

Rationale for this change

• Move and Rename function arrow::util::StringBuilder to internal namespace to avoid confusion with class arrow::StringBuilder

What changes are included in this PR?

• Move arrow::util::StringBuilder to arrow::internal::StringBuilder
• Rename arrow::internal::StringBuilder to arrow::internal::JoinToString
• Rename arrow/util/string_builder.{h|cc} to arrow/util/string_util_internal.{h|cc}

Are these changes tested?

Yes.

Are there any user-facing changes?

No. they are used internally.

• GitHub Issue: #46207

[Ziy1-Tan]

Thanks for tackling the work. Part of the current problem on CI is with calling the file *_internal.h . The header file won't be installed and this is used on some of the bindings which requires it. We can move the namespace to arrow::internal but keep installing the header file, probably naming the files string_util.cc and string_util.h?

Thank you for your review. Let me take a look.

[pitrou]

@github-actions crossbow submit -g cpp

[github-actions[bot]]

Revision: b9c5e4d0a0a5a4ac88ff033aab74586091c0222a

Submitted crossbow builds: ursacomputing/crossbow @ actions-44ea48c19a

Task	Status
example-cpp-minimal-build-static
example-cpp-minimal-build-static-system-dependency
example-cpp-tutorial
test-build-cpp-fuzz
test-conda-cpp
test-conda-cpp-valgrind
test-cuda-cpp-ubuntu-22.04-cuda-11.7.1
test-debian-12-cpp-amd64
test-debian-12-cpp-i386
test-fedora-39-cpp
test-ubuntu-22.04-cpp
test-ubuntu-22.04-cpp-20
test-ubuntu-22.04-cpp-bundled
test-ubuntu-22.04-cpp-emscripten
test-ubuntu-22.04-cpp-no-threading
test-ubuntu-24.04-cpp
test-ubuntu-24.04-cpp-bundled-offline
test-ubuntu-24.04-cpp-gcc-13-bundled
test-ubuntu-24.04-cpp-gcc-14
test-ubuntu-24.04-cpp-minimal-with-formats
test-ubuntu-24.04-cpp-thread-sanitizer

[conbench-apache-arrow[bot]]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit 1202d79fbafedfef77c5df2abd94abf2e3a7d06c.

There was 1 benchmark result indicating a performance regression:

• Commit Run on amd64-m5-4xlarge-linux at 2025-07-01 12:34:45Z
  • partitioned-dataset-filter (R) with dataset=dataset-taxi-parquet, language=R, query=vignette

The full Conbench report has more details.

[kou]

@github-actions crossbow submit test-r-rstudio-r-base-4.1-opensuse155

[github-actions[bot]]

Revision: b9c5e4d0a0a5a4ac88ff033aab74586091c0222a

Submitted crossbow builds: ursacomputing/crossbow @ actions-21818136da

Task	Status
test-r-rstudio-r-base-4.1-opensuse155