add a new cgraph++ library with a very basic AGraph C++ class

add a new cgraph++ library with a very basic AGraph C++ class

The AGraph class is an RAII-class that handles the lifetime of the
underlying Agraph_t C data structure. It currently supports
construction from DOT source, move construction and move assignment,
but not copy construction or copy assignment since the underlying C
data structure cannot be easily copied. It also supports retrieving a
non-owning pointer to the underlying Agraph_t.

The new C++ API is optional and can be enabled with the CMake option
"with_cxx_api".

Towards https://gitlab.com/graphviz/graphviz/-/issues/2001.