gst: deviceprodiver: Add fd property / camera portal support
From the main commit:
Add a `fd` property to `GstPipeWireDeviceProvider` so it can be used
with fds obtained from e.g. the camera xdg-desktop-portal.
This is needed so apps like Cheese or Camera can easily implement the
camera portal.
In order to make this work reliable the device provider needs to use a shared connection with the created sinks.