Terraform, GKE - add autoscaling Node Pools option

[aLekSer]

Is your feature request related to a problem? Please describe.

In order GKE Cluster to be more cost-efficient there should be an option to scale down GameServers Node Pool size.
Some of Agones users who used gcloud to provision their clusters has already autoscaling for Node Pools turned on. This functionality also is supported by Packed GameServerAllocation Scheduling.

Describe the solution you'd like
container_node_pool#autoscaling or
container_cluster#cluster_autoscaling

Currently we are using strict node_count parameter, which should be omitted when we are using autoscaling parameter.
So we could add this configuration separately to what we have now.

Describe alternatives you've considered
If it is possible merge both options together and depending on variable use autoscaling or strict node pool size.

Additional context
Using the Cluster Autoscaler with Agones - #368
and Kubernetes Node Autoscaling #145

[zmerlynn]

@chiayi is working on this but not a collaborator here so I can't assign.