Entry point for C++ Multi-dimensional Array API

Since GDAL 3.12, gdal_multidim_cpp.h is the include file for all headers related to the C++ Multi-dimensional Array API.

In earlier versions (and still available in the GDAL 3.x series), gdal_priv.h is the equivalent include file.

Note that the C++ API is also used by GDAL internals and may be subject from time to time to backwards incompatible changes.