Enable container to serve different handlers based on HANDLER_MODULE
environment variable, with live_serverless as the default.

Changes:
- Add load_handler() function with dynamic import logic
- Support nested module paths (e.g., handlers.custom.module)
- Maintain backward compatibility (defaults to live_serverless)
- Add comprehensive error handling with clear messages
- Include 15 new tests (11 unit, 4 integration)
- Test coverage for error scenarios and module switching

Usage:
  HANDLER_MODULE=custom_handler python handler.py

Implement HANDLER_MODULE env var for handler selection and HandlerEvent
protocol for standardized event structure across all handlers.

- New: handler_protocol.py with HandlerEvent/HandlerFunction
- Dynamic import based on HANDLER_MODULE (default: live_serverless)
- Backward compatible dict-to-HandlerEvent coercion