Refactor transform_lists_of_structs in row_operators.cu (#13288)

This split the functions `transform_lists_of_structs` in `row_operators.cu` into separate functions or simplified their implementation. From one function that can process both cases of having either one or two input columns, now we have two functions, each one processes one case.

Closes #13287.

Authors:
  - Nghia Truong (https://github.com/ttnghia)

Approvers:
  - Divye Gala (https://github.com/divyegala)
  - Vyas Ramasubramani (https://github.com/vyasr)
  - Mike Wilson (https://github.com/hyperbolic2346)
  - Bradley Dice (https://github.com/bdice)

URL: #13288