[REQ] [C++][Pistache] Make getXXX() return reference.

The getXXX() functions in the generated models for the pistache server always return data by value. This is rather wasteful, especially when large subobjects are being passed around.

I.e. currently generated code looks like:

class  Animal {
  public:
    std::string getName () const;
}

It would be much better to return a const reference instead. This would avoid making any copies. It also shouldn't break any existing code.

I.e. improved code would be:

class  Animal {
  public:
    std::string const & getName () const;
}

Start by locating the generator templates for Pistache server models and the tests covering generated C++ getters. Confirm how the current getXXX() declarations and definitions are produced, then verify that generated getters return const references without breaking existing generated-code tests.