[C++] #6501 - Problem when mapping a native type multiple times (#6514)

[C++] #6501 - Problem when mapping a native type multiple times (#6514)

* [C++] #6501 - Problem when mapping a native type multiple times
- idl.h:
added "native_type_pack_name"
- flatbuffers.h:
added CreateVectorOfNativeStructs variants which receive a pointer to the serialization function
- idl_gen_cpp.cpp:
adapted code generation in case "native_type_pack_name" attribute is present
- extended tests & docs; improved surrounding native_type docs a little

* integrated review feedback