Dynamically determine ICE agent's role

[mickel8]

ICE agent's role should be dynamically determined based on who sends the offer first and whether any side is running ice-lite. See: https://www.w3.org/TR/webrtc/#ref-for-dfn-icerole-1

So far, we have been always setting ICE agent's role to controlled and we were counting on ICE role conflict resolution.
However, some implementations does not support role conflict resolution and rely on correct role determination. See pion/ice#359 (comment)

In such case, we will get stuck and never establish ICE connection.

This PR, removes hard-coded agent's role and dynamically determines it according to W3C spec linked above.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.26%. Comparing base (880099e) to head (fc68977).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ex_webrtc/peer_connection.ex	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #205      +/-   ##
==========================================
- Coverage   88.30%   88.26%   -0.05%     
==========================================
  Files          49       49              
  Lines        2549     2565      +16     
==========================================
+ Hits         2251     2264      +13     
- Misses        298      301       +3     

Files with missing lines	Coverage Δ
lib/ex_webrtc/ice_transport.ex	80.00% <100.00%> (+3.07%)	⬆️
lib/ex_webrtc/sdp_utils.ex	91.01% <100.00%> (+0.05%)	⬆️
lib/ex_webrtc/peer_connection.ex	86.40% <87.50%> (-0.04%)	⬇️

... and 1 file with indirect coverage changes

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