Allow enabling Workload Identity in the GKE cluster

What does this MR do?

Allow enabling Workload Identity in the GKE cluster (disabled by default) so that pods can access GCP resources without having to configure a service account key.

Example use case: I'm configuring thanos-store for Prometheus in the staging-ref cluster, which needs access to a GCS bucket, and I would like to do so without configuring a service account key and using Workload Identity instead.

Related issues

https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14727

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up to date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
  • Documentation created/updated in the same MR.
  • If this MR adds an optional configuration - check that all permutations continue to work.
  • For Terraform changes: setup a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.