add the ability to use spot instances in gcp

[ramonskie]

as gcp supports spot prices voor vms it would be nice to implement this
https://cloud.google.com/compute/docs/instances/spot

as aws and alicloud also support this for a long time

it should also support a fallback as been done in aws cpi
cloudfoundry/bosh#915
cloudfoundry/bosh-aws-cpi-release@dedc281

spots can be managed by setting the ProvisioningModel
see https://pkg.go.dev/google.golang.org/api/compute/v1

it seems we already support preemptible, read diffrences below betwoon spot
https://sudipta-deb.in/2023/01/what-is-preemptible-and-spot-virtual-machine-in-google-cloud-platform.html

Spot Virtual Machines are the latest version of Preemptible Virtual Machines. However, Spot Virtual Machines provide new features that Preemptible Virtual Machines do not support.

Unlike Preemptible Virtual Machine, Sport Virtual Machine doesn’t have any runtime restrictions like 24 hours. It can run any number of hours.