Add set_role/2

[mickel8]

WebRTC determines agent's role based on who sends the offer first (https://w3c.github.io/webrtc-pc/#ref-for-dfn-icerole-1 and https://www.rfc-editor.org/rfc/rfc8445#section-6.1.1). Because an agent has to be initialized at the very beginning, its role can't be set in the constructor.

This PR adds an ability to set agent's role after it was created.

Things to note:

• adding remote candidate or gathering local candidates won't be possible until the role is set. This is to prevent creating checklist pairs as their priority rely on agent's role
• role can only be set once

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.17%. Comparing base (977359f) to head (f6249c2).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   83.99%   84.17%   +0.17%     
==========================================
  Files          25       25              
  Lines        1706     1719      +13     
==========================================
+ Hits         1433     1447      +14     
+ Misses        273      272       -1     

Files with missing lines	Coverage Δ
lib/ex_ice/ice_agent.ex	61.42% <100.00%> (+2.96%)	⬆️
lib/ex_ice/priv/ice_agent.ex	86.56% <100.00%> (+0.10%)	⬆️

... and 1 file with indirect coverage changes

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