Expose participant active event

[ladvoc]

This PR exposes the ParticipantActive event present in other SDKs, and documents when it is necessary to use it over ParticipantConnected. Example usage:

while let Some(ev) = room_rx.recv().await {
  let RoomEvent::ParticipantActive(participant) = ev else { continue };
  println!("Participant '{}' is active", participant.identity());
  // Send data streams or packets...
}  

[ladvoc]

Closing in favor of #688