ipam: netbox: implement deleting subnets

ipam: netbox: implement deleting subnets

Deleting a subnet did not delete any created entities in Netbox.
Implement deletion of a subnet by deleting all entities that are
created in Netbox upon creation of a subnet.

We are checking for any leftover IP assignments before deleting the
prefix, so we do not accidentally delete any manually created IP
assignments.

This method tries to check for any possible errors before editing the
entities. There is still a small window where external changes can
occur that lead to errors. We are touching multiple entities here, so
in case of errors users have to fix their Netbox instance manually.

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
Tested-by: Hannes Duerr <h.duerr@proxmox.com>
Link: https://lore.proxmox.com/20250310085103.30549-2-s.hanreich@proxmox.com
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>