webrtcstats: Improve selected candidate pair stats by adding ICE candidate info
requested to merge lshuying/gstreamer:gst-plugins-bad-improve_webrtcstats_with_selected_candidate_pair into main
Copied from https://gitlab.freedesktop.org//gstreamer/gst-plugins-bad/-/merge_requests/2423
The implementation follows w3.org specs:
- https://www.w3.org/TR/webrtc-stats/#icecandidate-dict*
- https://www.w3.org/TR/webrtc-stats/#candidatepair-dict*
Corresponding unit tests are also added.