Add candidate pairs stats

[mickel8]

This PR adds candidate pairs stats to the stats report. Not all fields from the W3C spec are available yet. They will be added when needed. This PR includes stats especially helpful in debugging ICE connectivity issues i.e. STUN binding requests/responses received/sent.

Relevant specs:

• W3C WebRTC sec. 8.19. https://www.w3.org/TR/webrtc-stats/#candidatepair-dict*

[codecov]

Codecov Report

Attention: Patch coverage is 76.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 88.42%. Comparing base (241931e) to head (e9ee2df).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ex_webrtc/dtls_transport.ex	40.00%	3 Missing ⚠️
lib/ex_webrtc/peer_connection.ex	85.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #203      +/-   ##
==========================================
+ Coverage   88.38%   88.42%   +0.04%     
==========================================
  Files          49       49              
  Lines        2531     2549      +18     
==========================================
+ Hits         2237     2254      +17     
- Misses        294      295       +1     

Files with missing lines	Coverage Δ
lib/ex_webrtc/dtls_transport.ex	83.45% <40.00%> (-1.95%)	⬇️
lib/ex_webrtc/peer_connection.ex	86.71% <85.00%> (+0.44%)	⬆️

... and 1 file with indirect coverage changes

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