scx_loader: Add rustland scheduler

[sirlucjan]

With the recent changes, scx_rustland has become quite a stable scheduler. Although there are more stable and sensible choices for the user, rustland also performs well.
Moreover, integrating scx_rustland into scx_loader will make it much easier to test new changes and pull requests.

lucjan at cachyos ~ 11:23:28    
❯ scxctl list          
supported schedulers: ["bpfland", "flash", "lavd", "p2dq", "tickless", "rustland", "rusty"]
lucjan at cachyos ~ 11:23:37    
❯ scxctl start -s rustland
started Rustland in Auto mode
lucjan at cachyos ~ 11:23:46    
❯ systemctl status scx_loader.service
● scx_loader.service - DBUS on-demand loader of sched-ext schedulers
     Loaded: loaded (/usr/lib/systemd/system/scx_loader.service; disabled; preset: disabled)
     Active: active (running) since Fri 2025-05-16 11:23:37 CEST; 14s ago
 Invocation: f7359caeab4247fcba159318c3fca94c
   Main PID: 87900 (scx_loader)
      Tasks: 22 (limit: 37394)
     Memory: 81.7M (peak: 82.2M)
        CPU: 114ms
     CGroup: /system.slice/scx_loader.service
             ├─87900 /usr/bin/scx_loader
             └─87964 scx_rustland

maj 16 11:23:37 cachyos systemd[1]: Starting DBUS on-demand loader of sched-ext schedulers...
maj 16 11:23:37 cachyos scx_loader[87900]: [INFO]: Starting as dbus interface
maj 16 11:23:37 cachyos systemd[1]: Started DBUS on-demand loader of sched-ext schedulers.
maj 16 11:23:46 cachyos scx_loader[87900]: [INFO]: starting Rustland with mode Auto..
maj 16 11:23:46 cachyos scx_loader[87900]: [INFO]: Got event to start scheduler!
maj 16 11:23:46 cachyos scx_loader[87900]: [INFO]: starting scx_rustland command
maj 16 11:23:46 cachyos scx_loader[87964]: 09:23:46 [INFO] RustLand scheduler attached

[arighi]

LGTM, thanks