Add deal.II example (with Kokkos)

[peterrum]

As suggested in https://github.com/CEED/libCEED/pull/1436#issuecomment-1919892567, this PR adds a deal.II example using Kokkos. The matrix-free infrastructure in deal.II that uses Kokkos is not as mature so that I only run BP1 (without overintegration) and BP5.