Add support for Identity Service in beta-private-cluster-update-variant

[vasrem]

TL;DR

Incorporate the feature introduced by this PR hashicorp/terraform-provider-google-beta#3957 into the beta-private-cluster-update-variant in order to be able to enable Identity Service using a flag similarly to enable_pod_security_policy flag.

Terraform Resources

* https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/container_cluster
* https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/container_cluster#nested_identity_service_config
* https://github.com/hashicorp/terraform-provider-google-beta/releases/tag/v4.6.0

Detailed design

I believe that this should be implemented similarly to https://github.com/terraform-google-modules/terraform-google-kubernetes-engine/blob/e6b928288cdd79035d32ea84fd8ce6ca40979246/modules/beta-private-cluster-update-variant/cluster.tf#L123-L128. This is a known pattern as far as I can tell from the code so I believe there should not be much to be added in the design. Let me know if there are questions and I'll try to answer them.

Additional information

No response