Refactor libcudf strings concatenate to use make_strings_children (#7937

)

Refactor combine.cu to split out `join_strings()` and enable `concatenate()` to use `make_strings_children` utility.

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Nghia Truong (https://github.com/ttnghia)
  - Jake Hemstad (https://github.com/jrhemstad)

URL: #7937

cpp/CMakeLists.txt

@@ -321,7 +321,8 @@ add_library(cudf
     src/strings/case.cu
     src/strings/char_types/char_cases.cu
     src/strings/char_types/char_types.cu
-    src/strings/combine.cu
+    src/strings/combine/concatenate.cu
+    src/strings/combine/join.cu
     src/strings/contains.cu
     src/strings/convert/convert_booleans.cu
     src/strings/convert/convert_datetime.cu