Add shell command media source drivers + misc improvements

[KW-M]

Description

Added feature:

• New drivers to allow the use of shell command output as a video or audio source. Added tests and example code for this driver. This can be used in cases where command line programs support media sources that pion/mediadevices doesn't yet (EG: libcamera based cameras), or to add filters to a video or audio stream before pion/mediadevices using ffmpeg or gstreamer.

Improvements:

• Added bit-rate control to vpx and h264 encoders.
• Bit-rate is auto-scaled based on the REMB bandwidth messages now.

Fixes:

• Possible deadlock when a driver throws a read error on the very first read (see the comment on the commit here for an example of this deadlock happening)

Reference issue

Workaround for #407

[at-wat]

@KW-M thank you for your contribution!
Could you split PRs per feature/improvement/fix?

[codecov]

Codecov Report

Base: 58.21% // Head: 54.59% // Decreases project coverage by -3.63% ⚠️

Coverage data is based on head (0bd945f) compared to base (f8f8511).
Patch coverage: 8.62% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #465      +/-   ##
==========================================
- Coverage   58.21%   54.59%   -3.63%     
==========================================
  Files          62       66       +4     
  Lines        3683     3953     +270     
==========================================
+ Hits         2144     2158      +14     
- Misses       1412     1666     +254     
- Partials      127      129       +2     

Impacted Files	Coverage Δ
pkg/codec/x264/x264.go	60.41% <0.00%> (-4.70%)	⬇️
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)
track.go	25.44% <33.33%> (+0.13%)	⬆️
pkg/codec/vpx/vpx.go	81.00% <65.00%> (-2.52%)	⬇️

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[KW-M]

Thanks! Will do.