Add support for workload_metadata_config in Standard GKE clusters

[Tirthankar17]

This PR is raised for this closed issue.

Currently the support for workload_metadata_config in GKE standard cluster is missing. We want to enable workload identity in the clusters and ensure that GKE Metadata server is enabled standard GKE clusters. The nodepool module does support workload_metadata_config but that is not unanimously used for all standard GKE clusters.

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

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