add support for racks and rack types

[matejv]

netbox 4.1 introduced support for rack types - these are models of racks, like device - device types.

Now you can create assets that represent racks and are related to rack types. So you can have your racks in inventory amongst all other assets.

One caveat: netbox supports creating racks without a rack type. You set all fields related to rack dimensions directly on a rack instead of a rack type. Unfortunately netbox-inventory always requires creating a rack type before you can create a rack from an asset. This requirement is deeply built into netbox-inventory and changing it would require massive changes which I don't want to make. So the limitation stays in place for now.