feat: Add comprehensive extension system for Pact CLI

🎯 Summary

This PR introduces a powerful extension system to the Pact CLI that enables seamless installation, management, and execution of additional tools and legacy binaries. The system supports both modern Rust-based extensions (like PactFlow AI) and traditional Ruby-based tools (pact-standalone), with intelligent version tracking and cross-platform compatibility.

✨ Key Features

Extension Management

• Install: Download and install extensions from remote sources
• List: Display available extensions with installed/latest version comparison
• Update: Update extensions to latest versions
• Uninstall: Remove extensions and clean up associated files
• Version Tracking: Show both installed and latest available versions

Command Integration

• Seamless Execution: Extensions work as first-class CLI commands
• PactFlow Integration: pact pactflow ai maps to pactflow-ai binary
• External Subcommands: Support for pact extension <name> syntax
• Dynamic Discovery: Automatically detects available pactflow commands from external crate

Platform Support

• Cross-Platform: macOS (Intel/Apple Silicon), Linux (x64/ARM64), Windows (x64/ARM64)
• Smart Binary Management: Handles platform-specific executables and archives
• Permissions: Automatic executable permissions on Unix systems

🔧 Implementation Details

Core Components

ExtensionManager - Central management class handling:

• Configuration persistence in ~/.pact/extensions/config.json
• Platform detection and URL generation
• Binary downloading with HTTP client (reqwest)
• Archive extraction (tar.gz/zip)
• Symlink creation for legacy tools

Extension Types:

• PactflowAi - Modern Rust binaries from download.pactflow.io
• PactRubyStandalone - Legacy Ruby tools from GitHub releases
• External - User-installed binaries

CLI Integration:

• New pact extension subcommand with full management capabilities
• Enhanced pact pactflow command supporting external extensions
• Async operation support using Tokio runtime

Technical Highlights

1. Dynamic Command Resolution: Inspects external crate subcommands to avoid hardcoding known commands
2. Version Intelligence:
   • PactFlow AI: Uses binary --version output parsing
   • Ruby Legacy: Records GitHub release tag during installation
   • Latest versions fetched from APIs in real-time
3. Robust File Management: Handles directories vs files, symlinks, and cleanup properly
4. Error Handling: Comprehensive error messages with helpful suggestions

Code Structure

src/cli/extension.rs          # Core extension management
├── ExtensionManager          # Main management logic
├── PlatformInfo             # Cross-platform support  
├── ExtensionConfig          # Configuration structures
└── CLI integration          # Subcommand handling

src/bin.rs                   # Enhanced main binary 
├── Extension command routing
├── PactFlow extension intercept
└── Dynamic command discovery

src/cli.rs                   # Updated CLI builder
├── Extension subcommand
├── PactFlow with extensions
└── External subcommand support

🧪 Testing

Verified Functionality

• ✅ Extension installation (pactflow-ai, pact-ruby-standalone)
• ✅ Version comparison and display
• ✅ Command execution (pact pactflow ai --help)
• ✅ Legacy tool symlink creation
• ✅ Configuration persistence
• ✅ Uninstallation and cleanup
• ✅ Cross-platform URL generation
• ✅ Error handling and user feedback

Example Usage

# Install extensions
pact extension install pactflow-ai
pact extension install pact-ruby-standalone

# List with version info
pact extension list
# Output:
# Name                 Type       Installed       Latest          Status    
# pactflow-ai          PactFlow AI 1.11.4         1.11.4          ✅ Installed
# pact-broker-legacy   Ruby Legacy v2.5.5         v2.5.5          ✅ Installed

# Use PactFlow AI seamlessly  
pact pactflow ai --help  # Maps to pactflow-ai binary

# Use legacy tools
pact extension pact-broker-legacy can-i-deploy --pacticipant my-app

📦 Dependencies Added

• reqwest = "0.12.0" - HTTP client for downloading binaries
• serde = "1.0" - Configuration serialization
• serde_json = "1.0" - JSON handling for config files
• home = "0.5.5" - Cross-platform home directory detection

🔄 Migration & Compatibility

• Backward Compatible: No breaking changes to existing CLI functionality
• Additive: All new features are opt-in via the extension subcommand
• Storage: Extensions stored in ~/.pact/extensions/ (configurable via PACT_CLI_EXTENSIONS_HOME)
• Integration: Works alongside existing pact-broker-cli, pact-plugin-cli, etc.

🎉 User Impact

This extension system bridges the gap between modern Pact tooling and legacy workflows, enabling users to:

1. Adopt New Tools: Seamlessly use PactFlow AI without complex installation
2. Maintain Legacy Support: Keep using Ruby-based tools with consistent interface
3. Unified Experience: Single CLI for all Pact operations regardless of underlying technology
4. Future-Proof: Easy adoption path for new Pact innovations

The system provides a foundation for the Pact CLI ecosystem to evolve while maintaining compatibility and ease of use.