Add AWS EC2 VM provider support

[jimpudar]

Summary

• Add a new aws-ec2 provider implementation with Terraform-backed provisioning, networking, and AWS config wiring
• Update provider factory, shared types, and rootcell instance flows to recognize and manage the new provider
• Extend integration coverage and add provider-specific docs and Nix/package updates needed for the new workflow

Testing

• Added/updated integration and unit coverage for provider selection, instance handling, and AWS EC2 provider behavior
• Not run (not requested)