MapPortHandler Callback for container support

[xinze-zheng]

Description

Expose a handler func(candidate Candidate) int handler that will be called upon addCandidate. The handler will set the mappedPort private field. During unmarshal, if mappedPort is not 0, it will replace the port in ICE candidate string.

Reference issue

https://github.com/pion/webrtc/issues/3155

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 87.01%. Comparing base (e1b445c) to head (460350f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #831      +/-   ##
==========================================
- Coverage   87.27%   87.01%   -0.27%     
==========================================
  Files          43       43              
  Lines        4819     4836      +17     
==========================================
+ Hits         4206     4208       +2     
- Misses        438      453      +15     
  Partials      175      175              

Flag	Coverage Δ
go	87.01% <100.00%> (-0.27%)	:arrow_down:

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[JoTurk]

@xinze-zheng I'm sorry about the internal API change, we had to fix the options so we can upgrade webrtc to use them.

[JoTurk]

I think it would be better if we can change the test to use the new Public api instead, NewAgentWithOptions, so it doesn't break if we change the internal API in the future, and so it matches the user contract. But this is up to you.