Add spotVM for GKE Node Pools

[modular-magician]

GKE Node Pools now support Spot VMs (not to be confused with Preemptible VMs):
https://cloud.google.com/kubernetes-engine/docs/concepts/spot-vms
This is not supported in web UI yet, but gcloud cli have it:
https://cloud.google.com/kubernetes-engine/docs/how-to/spot-vms#create_a_node_pool_with_enabled
API field spot: https://cloud.google.com/kubernetes-engine/docs/reference/rest/v1beta1/NodeConfig
Moved from: hashicorp/terraform-provider-google-beta#3833
part of #10309

Sorry, can't run acceptance tests. This has been tested via compiling terraform-provider-google-beta and we are using this custom compiled provider now to manage our spot vm node pools in gke.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

container: added `spot` field to `node_config` sub-resource (beta)

Derived from GoogleCloudPlatform/magic-modules#5443

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