feat: add Redis driver info for client identification

[vchomakov]

Description

Adds support for redis-py's DriverInfo class to identify fastapi-websocket-pubsub as an upstream driver in Redis connection metadata, following the Redis client library identification guidance.

This improves observability for Redis administrators by making fastapi-websocket-pubsub connections easily identifiable when inspecting connected clients (e.g., via CLIENT LIST or CLIENT INFO commands).

Changes

• Add _add_driver_info_to_broadcast() method to EventBroadcaster class
• Use DriverInfo class when available (modern redis-py)
• Fallback to lib_name/lib_version for older redis-py versions
• Add comprehensive unit tests in tests/test_driver_info.py

Motivation

Redis recommends that client libraries identify themselves using CLIENT SETINFO to help administrators understand which applications are connected. This change aligns fastapi-websocket-pubsub with this best practice by:

• Setting lib-name to identify fastapi-websocket-pubsub as the upstream driver (e.g., redis-py(fastapi-websocket-pubsub_v0.3.9))
• Setting lib-ver to the redis-py client version
• Making fastapi-websocket-pubsub connections distinguishable from other redis-py users

Compatibility

The implementation gracefully handles different redis-py versions:

• Modern redis-py: Uses DriverInfo class (preferred method)
• Older redis-py: Uses lib_name/lib_version fallback
• Non-Redis backends: Safely skips driver info injection

Testing

• Added 5 unit tests covering all code paths
• All existing tests pass
• Gracefully handles missing connection pools and non-Redis backends