String support for jcudf row to cudf column conversion (#10871)

This PR adds support for string column creation from jcudf row data. It leverages the fixed-width data copy to convert the offsets and lengths stored inside the fixed-width data section and then uses that information to copy the string data itself from the jcudf row format into the cudf column.

closes #10286

Authors:
   - Mike Wilson (https://github.com/hyperbolic2346)

Approvers:
   - Ray Douglass (https://github.com/raydouglass)
   - Nghia Truong (https://github.com/ttnghia)
   - https://github.com/nvdbaranec