Add Janus Streaming plugin client and integration tests

[danielmast]

Summary

• Implements JanusStreamingPlugin, a client for the Janus Streaming plugin
• Exports JanusStreamingPlugin, StreamingError, and StreamingEventType from the package
• Adds a full integration test suite in tests/test_plugin_streaming.py

What's included

janus_client/plugin_streaming.py

• Watch live/recorded streams from Janus mountpoints
• Full mountpoint lifecycle: create_mountpoint, destroy_mountpoint, edit_mountpoint, enable_mountpoint, disable_mountpoint
• Viewer operations: watch, pause, stop, switch
• Event system via on_event / StreamingEventType
• wait_webrtcup() to await WebRTC connection establishment
• Optional MediaRecorder integration for recording incoming streams

tests/test_plugin_streaming.py

• Integration tests covering both HTTP and WebSocket transports
• Tests: attach/destroy, list mountpoints, create/destroy, edit, enable/disable with secret protection, error handling for nonexistent mountpoints, event handler registration

Notes

The streaming plugin reverses the normal WebRTC signalling flow: Janus creates the SDP offer and the client creates the answer.