[FEA] Update GeneratedInternalRowToCudfRowIterator for string transitions

[mattahrens]

Is your feature request related to a problem? Please describe.
Update GeneratedInternalRowToCudfRowIterator. Be careful though this is the hard part because it does code generation to speed thing up. Also you might want to rename it because it is not going to CudfRow, it is going to a ColumnarBatch.

• This means we are going to need to have a version of copyData for the String that puts in the offset and length at the fixed length location and then writes the string data to the offset after the fixed width portion. Then updates a new offset value so later Strings can use it.

Ref: rapidsai/cudf#10033 (comment)

[amahussein]

reopened by #6367