Use offsetalator in gather_chars

[davidwendt]

Description

Update cudf::strings::detail::gather_chars() utility to use the offsetalator iterators.
The offsetalator enables the utility to use int32 or int64 offsets in the future with no additional changes.
This utility is mainly used by a make_strings_column factory and is already optimized for columns with both long and short strings.
The input_offsetalator is also updated to include an additional optional offset parameter for cases where the parent strings column has been sliced and requires building the iterator starting from a non-zero index within the offsets child column.

Checklist

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

[PointKernel]

LGTM

[davidwendt]

/merge