Add Kokkos backend

[masterleinad]

Builds on top of #13149 and tries to initialize Kokkos when needed and finalize it by registering Kokkos::finalize to std::atexit. In my tests that worked fine with Kokkos+Cuda and initial tests with Kokkos+Serial also looked
good.
It needs more thorough tests, though.

[bangerth]

I would prefer if the ensure_kokkos_initialized() function would simply call Assert(). I continue to think that we should initialize Kokkos in MPI_InitFinalize() or, alternatively, in a Kokkos_InitFinalize() class that one can use in non-MPI programs. If you then have your ensure_kokkos_initialized() function simply terminate, this is an indication that a program just needs to add the initialization by hand.

[masterleinad]

@bangerth At the moment, I am focussing on getting this pull request working with Kokkos+CUDA and DEAL_II_WITH_CUDA=ON. Then, I'm curious to see if calling std::atexit actually works with that configuration. If it does, I would consider it a long(er) term solution that could be done in a follow-up pull request, though. For now, it's at least helpful to automatically initialize Kokkos to be able to run the unit tests without modifying tests.

Of course, I'm open to other suggestions.

[masterleinad]

Superseded by #14510.