add support for virtual-disk data from netbox as ressource and data source

[joek-office]

Since netbox version 3.7 there is the ability to define virtual disks for virtual machine ressources as a dedicated object type.
This object type should be supported from this terraform provider to read out more than just the sum of the virtual disk sizes.

Affected Resource(s)

create new ressources and data sources:

• resource netbox_virtual_disk
• data_source netbox_virtual_disk

Expected Behavior

same as in the the other ressources and data sources. The provider has to be enhanced with this object type.

Actual Behavior

the object type virtual disk is not supported

References

https://github.com/netbox-community/netbox/releases/tag/v3.7.0
netbox-community/netbox#8356

[joek-office]

Seems work for this is currently in progress.
The underlying netbox client must be enhanced to support this new object type in netbox, but work is in progress:
fbreckle/go-netbox@1ddc00b