[visualizer] Add RTCP bitrate time series and packet overhead to total graphs

This change adds RTCP bitrate time series and dynamically calculated
packet overhead to both the total incoming and outgoing bitrate graphs,
controlled by a new command line flag: --include_overhead (defaulting to true).

Specifically:
- RTP and RTCP packet sizes are dynamically augmented with path-specific
  network-level overhead (IP/UDP/SRTP/STUN) matching their timeline.
- Both incoming and outgoing total bitrate calculations are refactored
  to process RTP/RTCP in a single combined sliding window loop.
- A command line flag --include_overhead is added to main.cc, with a
  description clarifying that it only affects total incoming and
  outgoing bitrate graphs.

Bug: None
Change-Id: I43473c7fd3ae3970dda71627aefea39f742f4371
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/477700
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47888}