[FEA] raft::matrix::slice to support row-major mdspans

[cjnolet]

This should be pretty straightforward to implement. It's possible to perform a slice on existing row-major data by constructing a column-major mdspan and then flipping the rows/cols in the slice coords but this is not particularly straightforward to the user and confusing to the user.

We should have a first-class row-major overload for it.