[Overlay] Add cli flag to register overlay as steamvr startup app

[TheButlah]

Prior art: SlimeVR/SlimeVR-Feeder-App@6b9263f

[TheButlah]

Also, prior rust art: https://github.com/alvr-org/ALVR/blob/8e9cc1700098b8b59bfec8c56390648ead78470b/alvr/launcher/src/commands.rs#L118