Added custom fields resource (VM's only)

[chapsuk]

Introducing custom_fields only for virtual machines instead of all resource

[fbreckle]

Ok I tried this and without custom fields this is now not annoying anymore :D
I noticed that, generally, if a custom field is introduced for a netbox type, that field keeps defaulting to an empty string if I do not set it in the resource

  # netbox_virtual_machine.testvm will be updated in-place
  ~ resource "netbox_virtual_machine" "testvm" {
      ~ custom_fields = {
          - "testfield" = "" -> null
        }
        id            = "1"
        name          = "my-test-vm"
        tags          = [
            "bar",
            "foo",
        ]
        # (10 unchanged attributes hidden)
    }

This means, that if a custom field is defined for a netbox type, to prevent this from happening, all terraform-managed resources of that type have to have this field set.

Since this happens only if a custom field is defined, I think this is okay.

Continuing this thought, maybe we can implement a provider-wide setting later that allows a terraform module to just ignore all custom fields or a subset. But this is not in scope for this MR.

Will review the rest later, I just wanted to check on the drift-detection spam.

[arjenvri]

Nice work! Would it possible to include custom fields support for sites as well ?

chapsuk#1

[fbreckle]

He made a MR with all the custom fields already at #143.
There were some issues, so we decided to start with VMs first and see how it goes. If all goes well, I definitely want to include custom fields for the other resources as well.