Add getters for ucxx::Header attributes

[pentschev]

Adding const getters for ucxx::Header attributes will make the class safer, preventing accidental modification of those attributes.

[wence-]

FWIW, I think this is a usual accepted pattern for get/set-style things:

struct Foo {
   private:
      int field_;
   public:
      void field(int f) { field_ = f; }
      const int &field() const { return field_; }
}