Add multiple execution client support with Geth, Erigon, and Reth

[jacobourar]

Pull Request Description

Multiple Execution Client Support

This PR adds support for multiple Ethereum execution clients, allowing users to easily switch between different implementations while maintaining compatibility with all consensus clients.

Added

New Execution Clients:

• Geth: Added configuration with proper JWT authentication and metrics
• Erigon: Added with BitTorrent sync support and configurable pruning
• Reth: Added Rust Ethereum implementation with special JWT handling

Documentation:

• Created INTEGRATION_GUIDE.md with detailed instructions for adding new clients
• Added CHANGELOG.md to track changes systematically
• Updated README with client-specific instructions and configuration notes

Configuration:

• Added EXECUTION_CLIENT environment variable for easy client selection
• Added client-specific resource limits and configuration options
• Implemented proper port mapping to avoid conflicts

Changed

• Restructured Docker Compose configuration for better maintainability
• Improved environment variable organization in .env-example
• Prometheus configuration with client-specific metrics endpoints
• Updated documentation to reflect the multi-client architecture

Fixed

• Resolved JWT authentication inconsistencies between clients
• Fixed port conflicts between execution clients
• Improved Reth configuration with proper entrypoint syntax
• Updated Docker image references to use the correct repositories

Testing

All execution clients have been tested with multiple consensus clients to ensure:

• Proper Engine API communication
• Correct metrics exposure
• Data persistence
• Resource limit enforcement

This implementation maintains backward compatibility while providing users with more flexibility in their node setup.