Add ice_gathering_state_change event

[mickel8]

No description provided.

[codecov]

Codecov Report

Merging #96 (0d593d0) into master (8cbead2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #96   +/-   ##
=======================================
  Coverage   88.43%   88.44%           
=======================================
  Files          32       32           
  Lines        1565     1566    +1     
=======================================
+ Hits         1384     1385    +1     
  Misses        181      181           

Files	Coverage Δ
lib/ex_webrtc/peer_connection.ex	85.35% <100.00%> (+0.02%)	⬆️

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