Add  DSCP settings

It's currently impossible to set traffic priority. I suspect it defaults to the lowest priority.

<img width="796" height="77" alt="Image" src="https://github.com/user-attachments/assets/6415b681-3684-42f0-acf2-45b4dc3099f5" />

flutter-webrtc has a networkPriority that controls DSCP (https://github.com/flutter-webrtc/flutter-webrtc/pull/1983).
It would be great to add similar settings for sip_ua
