Container_cluster: enables GKE workload metrics

[modular-magician]

As per https://cloud.google.com/blog/products/operations/managed-metric-collection-for-google-kubernetes-engine
GKE workload metrics are available in beta.

This updates the schema so that WORKLOAD is an allowed value for monitoring_config

This fixes hashicorp/terraform-provider-google#10275

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Release Note Template for Downstream PRs (will be copied)

container_cluster: Updated `monitoring_config` to accept `WORKLOAD`

Derived from GoogleCloudPlatform/magic-modules#5292