feat: add GCP Terraform deployment support

[supahcraig]

Summary

• Adds driver-redpanda/deploy/gcp/ with self-contained GCP Terraform configuration
• Provisions VPC, subnet, firewall rules, and compute instances (Redpanda brokers, clients, Prometheus)
• Supports optional VPC peering to an existing Redpanda BYOC network
• Includes terraform.tfvars.example and README with usage instructions for both standalone and BYOC deployments
• No changes to existing AWS infrastructure

Test plan

• terraform init && terraform apply provisions all instances cleanly
• ansible-playbook deploys successfully against fresh VMs
• Benchmark run completes against BYOC cluster
• terraform destroy tears down cleanly

[bfbarkhouse-redpanda]

LGTM