Add hybrid Slurm support to rrun with PMIx-based coordination

[pentschev]

This PR adds hybrid Slurm support for rrun, enabling RapidsMPF to run without MPI. This is achieved by reusing SlurmBackend that provides a passthrough mode.

The new hybrid Slurm execution model allows running one task per node, multiple GPUs per task with parent-mediated coordination:

• Root parent launches rank 0 first to obtain UCXX address, then broadcasts via PMIx to parents in all nodes
• rrun parents coordinate via PMIx then spawn non-root ranks with the root rank address

The hybrid execution mode requires specifying the number of tasks/ranks (-n) directly to rrun so it knows it should run in hybrid mode and know how many tasks/ranks per node it should launch.

Usage example:

  srun \
      --mpi=pmix \
      --nodes=2 \
      --ntasks-per-node=1 \
      --cpus-per-task=144 \
      --gpus-per-task=4 \
      --gres=gpu:4 \
      rrun -n 4 ./benchmarks/bench_shuffle -C ucxx

Requires #775

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[pentschev]

/ok to test