Is your feature request related to a problem? Please describe.
As GCP introduced a new generation of preemptible instances which is called Spot instance.
I'm trying to create a dataproc cluster with secondary workers as spot instances. But seems like in this line 778. There is no option to fulfill the request as gcloud cli does.

gcloud dataproc clusters create dummy \
 --region us-central1 \
 --zone us-central1-b \
...
 --num-workers 2 \
 --num-secondary-workers 0 \
 --secondary-worker-type=**spot** \
 --master-machine-type n1-standard-2 \
 --worker-machine-type n1-standard-4
...

Describe the solution you'd like
I think we can declare a new value to the Preemptibility enum which is SPOT and support parsing it to API body before making request.