Rework cudf::strings::detail::copy_range for offsetalator

[davidwendt]

Description

This reworks the cudf::strings::detail::copy_range() function to use the offsetalator instead of accessing the output offsets directly. Also refactored the code to remove the unnecessary template arguments. And added a benchmark to ensure these changes did not cause a performance impact.

Most of the code in cpp/include/cudf/strings/detail/copy_range.cuh was rewritten and moved to cpp/src/strings/copying/copy_range.cu.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.