Add codec ffmpeg

[3DRX]

This is a PR for adding ffmpeg based video encoders.

• Documentation, since ffmpeg library is introduced as dependency, building requires CGO so it's not a working-out-of-the-box experience.
• Update CI pipeline, or it will failed for not founding ffmpeg library. We add go-astiav as a dependency in pkg/codec/ffmpeg/go.mod to avoid breaking CI and other user's build. When using github.com/pion/mediadevices/pkg/codec/ffmpeg, it's a requirement for users to provide their own ffmpeg library as described here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.51%. Comparing base (20e8c50) to head (39bc14a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #621   +/-   ##
=======================================
  Coverage   44.51%   44.51%           
=======================================
  Files          80       80           
  Lines        5556     5556           
=======================================
  Hits         2473     2473           
  Misses       2913     2913           
  Partials      170      170           

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