feat: add Terraform GCP deployment for AGNTCY Directory

[anasmithdell]

Description

This PR adds Terraform infrastructure for deploying AGNTCY Directory on Google Cloud Platform with public federation support.

Infrastructure

• GKE Standard cluster with Workload Identity and Dataplane V2
• SPIRE server with web-mode federation and TLS certificates
• AGNTCY Directory stack with PostgreSQL, Zot registry, and P2P routing
• OIDC gateway for user authentication (Google OAuth)
• HTTP gateway with embedded AI Catalog UI
• ExternalDNS for automated DNS management
• cert-manager for Let's Encrypt TLS certificates

Usage

See terraform/environments/public-peering/README.md for deployment instructions.

Type of Change

• Bugfix
• [ x] New Feature
• Breaking Change
• Refactor
• Documentation
• Other (please describe)

Checklist

• [x ] I have read the contributing guidelines
• [x ] Existing issues have been referenced (where applicable)
• [x ] I have verified this change is not present in other open pull requests
• [x ] Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass