feat: add additional_tags support for standardized resource labeling

[mitch-hamm]

Summary

Adds additional_tags variable support across the GKE cluster module and submodules, enabling consistent GCP label propagation to all provisioned resources.

Root module (/):

• Added additional_tags variable (map(string), default {})
• module.gke + module.gke_private: pass cluster_resource_labels = var.additional_tags (labels the GKE cluster resource itself)
• local.node_pools_labels.all: merged with var.additional_tags (labels all node pool VMs)
• google_kms_crypto_key.gke_encryption_key: added labels = var.additional_tags

modules/dns-bucket:

• Added additional_tags variable
• Applied labels = var.additional_tags to google_storage_bucket (velero, tiered_storage, loki) and google_dns_managed_zone

modules/vpc:

• Added additional_tags variable
• Applied network_labels = var.additional_tags to module.network and labels = var.additional_tags to module.cloud_router

Use case

The CloudEnvironment reconciler sets cloud.streamnative.io/cloudenvironment-name and related labels via additionalTags in the Argo workflow TF vars. This change ensures those labels are propagated to GKE clusters, node pools, KMS keys, GCS buckets, DNS zones, VPC networks, and Cloud Routers.

Test plan

• Verify terraform plan shows label changes on existing resources (no destructive changes)
• Confirm additional_tags = {} (default) produces no diff on existing deployments
• Validate labels appear on GKE cluster, node pools, and GCS buckets after apply