T7827: Add unionfs-fuse package - #1036

add build unionfs-fuse

For context: unionfs-fuse is required for the legacy config backend that represents the running configuration and proposed configuration from user sessions as directories mounted via UnionFS, and it relies on subtle behavior details of UnionFS that make migration to OverlayFS or anything else very difficult, so we have long decided to just keep using UnionFS via FUSE until we completely replace the legacy config backend.

We aren't required to build this package from source yet, but it's removed from Debian Trixie, so when the time comes to update to Trixie we will need to start building it (assuming we don't get rid of the legacy configuration backend by then).

I think we should start building it now, we can always just remove this build script when the package is no longer needed.