Add Custom Fields for netbox_available_ip_address

[PlatformIpq]

As we need this for our development, it may be great to have here as well.
Sadly we encounter the same issue as #242 regarding irrelevant plans / changes with "" -> null:

=== NAME  TestAccNetboxAvailableIPAddress_deviceByFieldName
    resource_netbox_available_ip_address_test.go:264: Step 1/1 error: After applying this test step, the plan was not empty.
        stdout:


        Terraform used the selected providers to generate the following execution
        plan. Resource actions are indicated with the following symbols:
          ~ update in-place

        Terraform will perform the following actions:

          # netbox_available_ip_address.test will be updated in-place
          ~ resource "netbox_available_ip_address" "test" {
              ~ custom_fields       = {
                  - "custom_field_ip_address" = "" -> null
                }
                id                  = "68"
                # (7 unchanged attributes hidden)
            }

        Plan: 0 to add, 1 to change, 0 to destroy.

[xabinapal]

#667 should fix the permanent diff shown in testing.