[FEA] Add simple fractory methods to construct mdspan (and mdarray) objects from pointers and shape/stride information

[cjnolet]

This is just a placeholder to do this so that users that have a shape information and potentially a pointer can easily construct mdspan and mdarray objects:

auto arr = raft::create_mdspan(pointer, shape, stride);
auto arr = raft::create_mdarray(shape, stride);

Also thinking about matrix and vector and creation APIs:

auto arr = raft::create_vector(pointer, n);
auto arr = raft::create_vector(n);

auto arr = raft::create_matrix(pointer, nrows, ncols, rowmajor);
auto arr = raft::create_matrix(nrows, ncols, rowmajor);

[cjnolet]

This is being accomplished by #437