Add support for RTCP Receiver Reports

[LVala]

This PR only adds RTPReceiver.ReportRecorder module, akin to PeerConnection.TWCCRecorder.
The actual functionality of sending the reports, as well as RTPSender.ReportRecorder for SenderReports, will be implemented in subsequent PRs.

[codecov]

Codecov Report

Merging #77 (ee60139) into master (682c434) will increase coverage by 0.37%.
The diff coverage is 98.11%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   88.43%   88.81%   +0.37%     
==========================================
  Files          29       30       +1     
  Lines        1306     1359      +53     
==========================================
+ Hits         1155     1207      +52     
- Misses        151      152       +1     

Files	Coverage Δ
lib/ex_webrtc/rtp_receiver/report_recorder.ex	98.11% <98.11%> (ø)

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[mickel8]

Really nice code! 👍 Clean, concise and easy to understand. Also, very nice comment in the ReportRecorder 👀

I have some questions regarding fraction_lost and total_lost 🤔

[mickel8]

We should count in duplicate packets here?

This number is
      defined to be the number of packets expected less the number of
      packets actually received, where the number of packets received
      includes any which are late or duplicates.  Thus, packets that
      arrive late are not counted as lost, and the loss may be negative
      if there are duplicates.

[LVala]

Here I was just following what Pion does, if we wanted to include duplicate packets, then the whole lost_packets MapSet is unnecessary, we could just count the number of packets received. To be honest, I'm not really sure why is that, I just assumed that we shouldn't count duplicate packets, but I'll look into this more.