Add shell command media source drivers

[KW-M]

Description

New drivers to allow the use of shell command output as a video or audio source (+ tests and example code).

• This can be used in cases where command line programs support media sources that pion mediadevices doesn't yet
• Examples: using libcamera based cameras with libcamera-video, or to add filters to a video or audio stream before pion mediadevices using ffmpeg or gstreamer.

Reference issue

Workaround for #407

• People using a Raspberry Pi can use a shell command like the following as a video input source with frame format I420
  libcamera-vid --width 640 --height 480 --framerate 16 --codec yuv420 --flush 1 --timeout 0 --nopreview 1 --output -
• This PR does not address new for hardware encoding on the Raspberry Pi, however non-hardware encoders h264 & vp8/vp9 work fine for now.

[codecov]

Codecov Report

Base: 58.21% // Head: 54.84% // Decreases project coverage by -3.38% ⚠️

Coverage data is based on head (1d228e0) compared to base (f8f8511).
Patch coverage: 0.86% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #468      +/-   ##
==========================================
- Coverage   58.21%   54.84%   -3.38%     
==========================================
  Files          62       66       +4     
  Lines        3683     3913     +230     
==========================================
+ Hits         2144     2146       +2     
- Misses       1412     1640     +228     
  Partials      127      127              

Impacted Files	Coverage Δ
pkg/driver/cmdsource/audiocmd.go	0.00% <0.00%> (ø)
pkg/driver/cmdsource/videocmd.go	0.00% <0.00%> (ø)
pkg/frame/framesize.go	0.00% <0.00%> (ø)
pkg/driver/cmdsource/cmdsource.go	3.33% <3.33%> (ø)

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[KW-M]

I'm not quite sure why the test cases for audiocmd.go and videocmd.go don't show up in Codecov. I'm fairly new to testing in go / testing in general, so any pointers would be much appreciated.

[at-wat]

I'm not quite sure why the test cases for audiocmd.go and videocmd.go don't show up in Codecov. I'm fairly new to testing in go / testing in general, so any pointers would be much appreciated.

https://github.com/pion/mediadevices/actions/runs/3964689614/jobs/6793835936#step:5:259

audiocmd_test.go:63: ffmpeg command not found in path. Skipping test. Err:  exec: "ffmpeg": executable file not found in $PATH