webrtcstats: Improve selected candidate pair stats by adding ICE candidate info
requested to merge lshuying/gst-plugins-bad:improve_webrtcstats_with_selected_candidate_pair into master
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.