Use --external-launcher for Slurm launch

[mattaezell]

The Slurm srun --external-launcher option, added in 23.11, allows users to start a daemon that has access to all the resources without actually consuming any of them. It was intended for process managers like hydra, orted, or flux-as-a-step-manager to launch tasks, but I think we can use it here also.

The configury need more work, but I wanted to get some feedback before spending time on that. Nothing older than 23.11 is considered "supported" anymore. Does it make sense to completely remove the BROKEN_SRUN construct at this point?

ezy@borg005:~/mpi_hello> time spindle srun -n2240 -N40 ./mpihi >/dev/null

real    0m6.724s
user    0m0.021s
sys     0m0.054s
ezy@borg005:~/mpi_hello> time srun -n2240 -N40 ./mpihi >/dev/null

real    0m11.810s
user    0m0.015s
sys     0m0.044s

[mplegendre]

I hadn't been aware of the --external-launcher before, but a quick scan of the man page is promising. That is the kind of thing we're trying to do when we launch spindle_be. Will investigate further.