Local rtcp-fb values are used when answering

[digitalix]

Description

Currently, pion uses remote rtcp-fb when answering.
According to https://tools.ietf.org/id/draft-ietf-rtcweb-sdp-08.html a=rtcp-fb lines should list supported feedback mechanisms. This change makes pion use rtcp-fb values provided by the user instead of using whatever was given by the remote pc.

[codecov]

Codecov Report

Merging #1968 (b04c34e) into master (dce9704) will increase coverage by 0.05%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master    #1968      +/-   ##
==========================================
+ Coverage   76.96%   77.02%   +0.05%     
==========================================
  Files          85       85              
  Lines        6213     6224      +11     
==========================================
+ Hits         4782     4794      +12     
  Misses       1026     1026              
+ Partials      405      404       -1     

Flag	Coverage Δ
go	78.80% <90.90%> (+0.05%)	⬆️
wasm	70.72% <ø> (ø)

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Impacted Files	Coverage Δ
mediaengine.go	90.75% <85.71%> (-0.12%)	⬇️
sdp.go	87.39% <100.00%> (+0.14%)	⬆️
icetransport.go	68.63% <0.00%> (+1.18%)	⬆️

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