External references for the object API thru a resolver function.

This allows hashed string fields to be used for lookup of any
C++ objects, a pointer to which are then stored in the object
besides the original hash for easy access.

Change-Id: I2247a13c349b905f1c54660becde2c818ad23e97
Tested: on Linux.
Bug: 30204449

include/flatbuffers/idl.h

@@ -450,6 +450,7 @@ class Parser : public ParserState {
     known_attributes_["csharp_partial"] = true;
     known_attributes_["streaming"] = true;
     known_attributes_["idempotent"] = true;
+    known_attributes_["cpp_type"] = true;
   }
 
   ~Parser() {