core: frontend: camera manager: allow auto-disabling video streams when using WebRTC

### Current behaviour

Configured UDP/RTSP video streams are persistent, even while the video is _also_ being actively streamed over WebRTC (e.g. to Cockpit).

### Expected or desired behaviour

1. Add a per-stream "extra configuration" option to "disable primary stream while WebRTC is active"
2. (Maybe) add a global setting to determine the default value of that option for newly created streams

@joaoantoniocardoso has mentioned this may require an extra command-line option in MCM

### Prerequisites

- [X] I have checked to make sure that a similar request has not already been filed or fixed.