Describe the enhancement requested

During review of #46180, it came up that Arrow C++ has two StringBuilders:

1. The public builder for utf8() arrays, https://github.com/apache/arrow/blob/main/cpp/src/arrow/array/builder_binary.h#L424 (class)
2. A helper in https://github.com/apache/arrow/blob/main/cpp/src/arrow/util/string_builder.h (function)

We should,

• Move StringBuilder (2) in cpp/src/arrow/util/string_builder.h from arrow::util to arrow::internal to make its intended use more clear
• Rename arrow/util/string_builder.{h,c} and arrow::util::StringBuilder to something else to avoid confusing humans and computers

No changes would be made to the official Arrow StringBuilder class.

Component(s)

C++