Add an RTX-associated TrackLocal

[kevmo314]

Description

Exposes the TrackLocal that has been configured as the RTX track.

Note: This PR is missing the actual transmission part and only exposes the configured RTX track. Working on that part next.

Reference issue

See https://github.com/pion/webrtc/pull/2592

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 65.11%. Comparing base (661a92f) to head (6e00d5a).

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HEAD has 2 uploads less than BASE

Flag	BASE (661a92f)	HEAD (6e00d5a)
go	2	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2852       +/-   ##
===========================================
- Coverage   79.04%   65.11%   -13.94%     
===========================================
  Files          89       67       -22     
  Lines        8477     3262     -5215     
===========================================
- Hits         6701     2124     -4577     
+ Misses       1292     1011      -281     
+ Partials      484      127      -357     

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[Sean-Der]

Thank you @kevmo314! I cherry picked things and continuing it here https://github.com/pion/webrtc/pull/2676