Add API to trigger data disk move only

[agners]

Add a new D-Bus API which allows to trigger the data disk move only. This allows to handle the selection and partitioning from Supervisor directly (via UDisks2 D-Bus API). Handling partitioning directly in Supervisor should improve device detection/selection and allows for better error handling.

Before calling this API, make sure to prepare a data disk using the GPT partition layout and with the partition label (not file system label) of hassos-data-external. Then call the API without argument and reboot the system.

# busctl --verbose call io.hass.os /io/hass/os/DataDisk io.hass.os.DataDisk MarkDataMove

[pvizeli]

We should consider to only return *dbus.Error or nil - there is no need for return bool

[agners]

Right, I was somehow under the impression that D-Bus (or the Go library we use for D-Bus) requires a return value, but just tested, that seems not to be the case.

I'll adjust the new API.