Add Terraform files to deploy Envoy RateLimiter

[tarun-google]

Terraform files to deploy envoy ratelimiter to GKE.

Features:

1. Private GKE cluster with Autopilot
2. Static Ip for ratelimit service loadbalancer
3. pod HPA for ratelimit service.

GitHub Actions Tests Status (on master branch)

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[gemini-code-assist]

Summary of Changes

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This pull request delivers a complete Infrastructure-as-Code solution using Terraform to deploy a robust and scalable Envoy Rate Limit Service on Google Cloud's GKE Autopilot. It streamlines the setup of a private Kubernetes environment, including necessary networking, the rate limiting application with its Redis backend, and automatic scaling capabilities, making it easier for users to integrate rate limiting into their GCP-based applications.

Highlights

• Terraform Module for Envoy RateLimiter: Introduces a new, production-ready Terraform module to deploy a scalable Envoy Rate Limit Service on Google Kubernetes Engine (GKE) Autopilot.
• Private GKE Autopilot Cluster: Configures a private GKE Autopilot cluster, ensuring nodes have internal IPs only and leveraging Cloud NAT for external connectivity.
• Static Internal Load Balancer IP: Provisions a static internal IP address for the Rate Limit service's load balancer, making it accessible within the VPC.
• Horizontal Pod Autoscaling (HPA): Implements HPA for the Rate Limit service, allowing it to automatically scale based on CPU and memory utilization.
• Integrated Redis and Metrics: Deploys Redis for rate limit counters and includes a StatsD Exporter sidecar for Prometheus-compatible metrics.
• Comprehensive Documentation: Provides a detailed README.md with architecture overview, prerequisites, deployment instructions, testing steps, and variable descriptions.

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