Add example that interfaces with external kernel frameworks

[MarcelKoch]

Our custom-matrix-format shows how users can use their own kernels together with our library. But it is restricted to an openmp and cuda kernel, which are implemented separately. It would be good to also have an example (or replace the current one) which shows how to integrate ginkgo with kernel frameworks such as Kokkos, Raja, ... This might also be helpful in cases like the discussion #1190.