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

[raulcd]

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?

[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]

Thanks for doing this. Just one suggestion, otherwise LGTM.

[pitrou]

The detail namespace is now superfluous given there's already an enclosing internal namespace.