[FEA] Implement a function to convert a list of strings columns into a structs column with desired schema

[ttnghia]

In GpuJsonToStruct, after reading the input JSON using cudf::read_json, we need to convert the output table (consisting of many strings columns) into the final structs column with the desired schema. Currently, that step is done in Scala code, which can have a lot of overhead of unnecessarily copying the intermediate columns into the final output column. We can do that better in native C++ code, performing the conversion if needed while just moving the intermediate columns into the output column without any copying.