Add devcontainer configuration for development

[ktamas77]

This PR adds a complete development container configuration that makes it easy for contributors to start developing without manual setup.

Changes

• Added .devcontainer/devcontainer.json with all required dependencies:
  • Bun (for formatting and scripts)
  • Terraform (for module development)
  • Docker-in-Docker (for running tests with --network=host)
  • VS Code extensions (Terraform, Bun, Prettier)
• Added .devcontainer/README.md with usage instructions
• Automatic dependency installation on container creation
• Pre-configured VS Code settings matching repository standards

Benefits

Contributors can now:

• Open the repo in VS Code and click "Reopen in Container"
• Use GitHub Codespaces without any setup
• Use Coder's docker-devcontainer template
• Start developing immediately with all dependencies ready

Testing

The devcontainer has been configured to:

• Run bun install on creation
• Mount Docker socket for test support
• Use --network=host as required by Terraform tests
• Include all extensions mentioned in CONTRIBUTING.md

/claim #51