[Feature]: add provisioningKey to equinix_network_device VendorConfiguration schema

[ocobles]

provisioningKey seems to be mandatory to deploy new Zscaler devices, at least from the portal. However, it is not available in terraform and it is not described in the API specs https://developer.equinix.com/catalog/network-edgev1#operation/createVirtualDeviceUsingPOST but it is in https://developer.equinix.com/catalog/network-edgev1#operation/restoreDeviceBackupUsingPATCH

terraform-provider-equinix/equinix/resource_network_device.go

Line 867 in d63009a

func createVendorConfigurationSchema() map[string]*schema.Schema {