Optional-ness in reflection (#6097)

* Optional scalars in reflection

* fixed name collision

* Remove code duplicated by merge

Co-authored-by: Casper Neo <cneo@google.com>

reflection/reflection.fbs

@@ -74,6 +74,7 @@ table Field {
     key:bool = false;
     attributes:[KeyValue];
     documentation:[string];
+    optional:bool = false;
 }
 
 table Object {  // Used for both tables and structs.