Live indicator

[cerahmed]

As a long-term user of WebRTC camera in my own deployment, I've always needed a live indicator to show if the rtc stream is actually live. This is especially crucial for me as I control garage and main doors while looking at camera streams, and I need to ensure the opening/closing commands are executed correctly.

Here's a summary of this pull request:

1. Added a new option in the card config live_indicator that defaults to false.
2. When live_indicator is true, it adds a little dot to the header which turns green when video is live, and red when video is lagging/disconnected.
3. The live indicator dot is updated using an interval that checks if the videos's last rendered frame timestamp is 500ms or less compared to the current timestamp.