Add support for VP8 simulcast

[mickel8]

This PR moves and adjusts VP8 simulcast code from Membrane RTC Engine

[codecov]

Codecov Report

Attention: Patch coverage is 76.08696% with 11 lines in your changes missing coverage. Please review.

Project coverage is 84.97%. Comparing base (4cebfd8) to head (3bc96f7).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ex_webrtc/rtp/munger.ex	58.82%	7 Missing ⚠️
lib/ex_webrtc/rtp/vp8.ex	0.00%	3 Missing ⚠️
lib/ex_webrtc/rtp/vp8/munger.ex	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
- Coverage   85.29%   84.97%   -0.32%     
==========================================
  Files          49       51       +2     
  Lines        2584     2629      +45     
==========================================
+ Hits         2204     2234      +30     
- Misses        380      395      +15     

Files with missing lines	Coverage Δ
lib/ex_webrtc/rtp/vp8/munger.ex	96.15% <96.15%> (ø)
lib/ex_webrtc/rtp/vp8.ex	0.00% <0.00%> (ø)
lib/ex_webrtc/rtp/munger.ex	84.78% <58.82%> (-15.22%)	⬇️

... and 3 files with indirect coverage changes

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