[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>

tests/key_field_test.h

@@ -5,6 +5,10 @@ namespace flatbuffers {
 namespace tests {
 
 void FixedSizedScalarKeyInStructTest();
+void StructKeyInStructTest();
+void NestedStructKeyInStructTest();
+void FixedSizedStructArrayKeyInStructTest();
+
 
 }  // namespace tests
 }  // namespace flatbuffers