Add streaming handlers supporting

[vadikko2]

🚀 New Features

Streaming Support & SSE

• StreamingRequestMediator: Added a new mediator designed for handling streaming requests that yield results incrementally.
• StreamingRequestHandler: Introduced a new handler type for processing large batches or long-running operations with real-time progress updates.
• FastAPI SSE Integration: Added native support and examples for using StreamingRequestMediator with Server-Sent Events (SSE) in FastAPI applications.

Parallel Event Processing

• Concurrent Event Handling: Both RequestMediator and StreamingRequestMediator now support processing domain events in parallel.
• Concurrency Control: Added max_concurrent_event_handlers parameter to limit the number of simultaneously running event handlers.
• Configuration: Added concurrent_event_handle_enable flag to toggle between sequential and parallel execution.

Dependency Injection

• dependency-injector Support: Added explicit support and documentation for the dependency-injector library using the DependencyInjectorCQRSContainer adapter.

📚 Documentation

• Updated README with comprehensive examples for:
• Streaming Request Handlers.
• Parallel Event Processing configuration.
• FastAPI SSE implementation.
• DI container setup (both di and dependency-injector).