Allow passing node service_account when autopilot enabled

[modular-magician]

Removes the conflict between cluster_autoscaling and autopilot making it possible to supply a service_account:

cluster_autoscaling.auto_provisioning_defaults.service_account

This change also makes cluster_autoscaling.enabled optional to simplify passing args when autopilot is enabled.

As a result it is now possible to pass false when creating autopilot clusters but this will be caught by server side validaton.

fixes hashicorp/terraform-provider-google#9505

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Release Note Template for Downstream PRs (will be copied)

container: fixed a bug where `cluster_autoscaling.auto_provisioning_defaults.service_account` can not be set when `enable_autopilot = true` for `google_container_cluster`

Derived from GoogleCloudPlatform/magic-modules#6733