Expose /sys/class/dmi/id/chassis_asset_tag in varlink/dbus commonly used
by companies to track inventory such as laptops.
On desktops and other products the `chassis_asset_tag` can contain
rubbish similar to product_name/product_vendor.
Closes: #36442
The DMI rules where so far guarded by an ACTION=="add" rule, but that
doesn't really make sense for setting properties (only for setting
access modes/ownership of nodes).
Hence let's move this into its own file, that guards properly on
ACTION!="remove".
Before this change the hardware vendor/model info would be dropped
whenever the device was retriggered.