feat: Define a device for an arbitrary mountpoint

[moeenio]

Is your feature request related to a problem? Please describe. I transfer music to an iOS device using KIO AFC. Cantata doesn't detect that as a device. I would however like to use Cantata's sync and transcoding features with it.

Describe the solution you'd like It would be great to be able to add an arbitrary directory (mount point) as a device.

Describe alternatives you've considered Have Cantata detect all KIO devices

Additional context i love cantata tysm for maintaining it