[cpp] Json parsing: adding support for parsing nested lists and top level lists (#4338)

* Extended json parsing capability: add support for parsing nested lists and top level lists

* Stylistic conformance with surrounding code + generalized comments

* More code tidy-up for stylistic conformance with surrounding code

* Blank lines

* Reverted changes related to top-level list parsing

* Styling: newline before else

* Taking out ProcessTableFields which is no longer needed as the top level list change was reverted.