Initial config for OpenMPI 5 on Slingshot

[mszpindler]

TO DO:

• Verify ROCm support
• Work on native srun interaction (with --mpi=pmix, without mpiexec)

[klust]

To my understanding, Open MPI also needs a newer version of the Slingshot software stack than we currently have on LUMI, so it may not be fully operational until after the long awaited mother-of-all-updates.

[mszpindler]

This only works on a single node over ofi:cxi with --network=single_node_vni slurm option OR one task per multiple nodes with plm_slurm_args="--exact --overlap" prte runtime option.