Add PeerConneciton.set_packet_loss

[mickel8]

This PR adds an ability to set very simple packet loss for a peer connection.

While I haven't seen such function in any other WebRTC implementation, I found it useful for experimental and demo purposes where setting packet loss on a network interface might be too broad.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 88.09%. Comparing base (2ef1582) to head (04be4ae).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ex_webrtc/peer_connection.ex	0.00%	2 Missing ⚠️
lib/ex_webrtc/dtls_transport.ex	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
+ Coverage   87.81%   88.09%   +0.28%     
==========================================
  Files          47       47              
  Lines        2371     2378       +7     
==========================================
+ Hits         2082     2095      +13     
+ Misses        289      283       -6     

Files with missing lines	Coverage Δ
lib/ex_webrtc/dtls_transport.ex	84.67% <92.30%> (+0.58%)	⬆️
lib/ex_webrtc/peer_connection.ex	85.88% <0.00%> (+0.33%)	⬆️

... and 4 files with indirect coverage changes

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