[C++] Enable using struct and array of struct as key (#7741)

* add unit tests for support struct as key

* make changes to parser and add helper function to generate comparator for struct

* implement

* add more unit tests

* format

* just a test

* test done

* rerun generator

* restore build file

* address comment

* format

* rebase

* rebase

* add more unit tests

* rerun generator

* address some comments

* address comment

* update

* format

* address comment

Co-authored-by: Wen Sun <sunwen@google.com>
Co-authored-by: Derek Bailey <derekbailey@google.com>