mdspan: Kokkos implementation

[EnricoDeg]

Try to use mdspan original implementation from Kokkos (https://github.com/kokkos/mdspan) to use the memory view with HIP or with CPU code before it gets part of the standard (avoid experimental implementation in nvhpc to allow also testing)

[EnricoDeg]

I suggest to have an external folder and use the mdspan repo as a submodule