fix: replace int64 with NestedCable in NestedPowerPort

[aluki]

I get this error:

Error: json: cannot unmarshal object into Go struct field NestedPowerPort.power_port.cable of type int64
│
│   with netbox_device_power_outlet.p1,
│   on main.tf line 102, in resource "netbox_device_power_outlet" "p1":
│  102: resource "netbox_device_power_outlet" "p1" {

The NestedPowerPort model defines the Cable as int64. Changed to NestedCable, as the API returns the cable JSON, not only the id.