Add support for RTCP Sender Reports

[LVala]

This PR adds RTPSender.ReportRecorder module, akin to RTPReceiver.ReportRecorder and PeerConnection.TWCCRecorder.

[codecov]

Codecov Report

Merging #78 (14e791a) into master (682c434) will increase coverage by 0.16%.
Report is 1 commits behind head on master.
The diff coverage is 96.42%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
+ Coverage   88.43%   88.60%   +0.16%     
==========================================
  Files          29       30       +1     
  Lines        1306     1334      +28     
==========================================
+ Hits         1155     1182      +27     
- Misses        151      152       +1     

Files	Coverage Δ
lib/ex_webrtc/rtp_sender/report_recorder.ex	96.42% <96.42%> (ø)

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[mickel8]

I would add a comment here, sth like

A packet is in order when it is from the next cycle, or current cycle with delta > 0

In ReceiverReports we have:

    cycle =
      cond do
        delta in -@breakpoint..@breakpoint -> last_cycle
        delta < -@breakpoint -> last_cycle + 1
        delta > @breakpoint -> last_cycle - 1
      end

which is really nice and easy to to read but if someone didn't see this code, it might be pretty hard to get what's going on?