Use offsetalator in cudf::io::json::detail::parse_string

[davidwendt]

Description

Updates the cudf::io::json::detail::parse_string function to use the offsetalator for building a strings column instead of size_type pointers. The output row sizes are computed in the first pass through the kernels and then converted to offsets. The offsets are wrapped with an offsetalator on the 2nd pass to locate each individual rows' output position in the chars data.

Checklist

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

[GregoryKimball]

Large strings in the JSON reader - here we come!! eventually ;)

[mhaseeb123]

Looks good to me. Thank you for catching the std::cout in test

[karthikeyann]

LGTM 👍

[davidwendt]

/merge