[C++][Ue4] various bus fixes (#6539)

* Fixed compilation on linux and UE4.23

* Added string and enum type definition support

* Better handling or variable names, should avoid conflicts with reserved keywords or empty variable names in edge cases

* Updated samples

samples/client/petstore/cpp-ue4/OpenAPI.Build.cs

@@ -26,5 +26,6 @@ public class OpenAPI : ModuleRules
                 "Json",
             }
         );
+        PCHUsage = PCHUsageMode.NoPCHs;
     }
 }