webrtcbin: small stats improvements

Author: Jan Schmidt jan@centricular.com Date: Tue Feb 7 04:58:04 2023 +1100

webrtc: Calculate the jitter for remote-inbound-rtp stats

Populate the clock-rate in the internal stats structure, so
it can be used by the _get_stats_from_remote_rtp_source_stats()
method to calculate remote receivers' jitter.

Author: Jan Schmidt jan@centricular.com Date: Wed Feb 1 10:44:26 2023 +1100

webrtcbin: Report full codec-stats for source pads

Use the current caps for webrtcbin srcpads, as received_caps
are only stored for sink pads based on incoming caps events.

Makes it so that webrtcbin stats reports contain fuller
codec information.