feat: add slurm_bin_prefix for non-standard Slurm binary paths

[kochanha]

Summary

• Some Slurm clusters (e.g. evgl1) don't have sinfo/squeue/scontrol in the remote shell PATH, causing ksh: sinfo: not found errors when running commands via SSH
• Added configurable slurm_bin_prefix per Slurm cluster to specify the full binary directory path (e.g. /opt/slurm-21.08/bin)
• Clusters without the setting continue to use bare commands as before

Changes

• models.py: Added slurm_bin_prefix field to SlurmClusterConfig
• slurm.py: Added slurm_bin_prefix parameter to collect_slurm_snapshot(), prefixes sinfo/squeue/scontrol with the configured path
• config.py: Parse slurm_bin_prefix from config JSON
• cli.py: Prompt for slurm_bin_prefix during opensmi onboard, display in review, pass through to runtime

Usage

In opensmi.json:

{
  "name": "evgl1",
  "login_node": "evgl1",
  "user": "ckim91",
  "slurm_bin_prefix": "/opt/slurm-21.08/bin"
}

Or enter the path interactively during opensmi onboard.

Test plan

• Verify opensmi slurm works on clusters without slurm_bin_prefix (no regression)
• Verify opensmi slurm works on evgl1 with slurm_bin_prefix: /opt/slurm-21.08/bin
• Verify opensmi onboard prompts for Slurm bin path and saves it correctly