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[bot]]

Codecov Report

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

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

```diff @@ 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](https://codecov.io/gh/pion/mediadevices/pull/468?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | Coverage Δ | | |---|---|---| | [pkg/driver/cmdsource/audiocmd.go](https://codecov.io/gh/pion/mediadevices/pull/468?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#diff-cGtnL2RyaXZlci9jbWRzb3VyY2UvYXVkaW9jbWQuZ28=) | `0.00% <0.00%> (ø)` | | | [pkg/driver/cmdsource/videocmd.go](https://codecov.io/gh/pion/mediadevices/pull/468?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#diff-cGtnL2RyaXZlci9jbWRzb3VyY2UvdmlkZW9jbWQuZ28=) | `0.00% <0.00%> (ø)` | | | [pkg/frame/framesize.go](https://codecov.io/gh/pion/mediadevices/pull/468?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#diff-cGtnL2ZyYW1lL2ZyYW1lc2l6ZS5nbw==) | `0.00% <0.00%> (ø)` | | | [pkg/driver/cmdsource/cmdsource.go](https://codecov.io/gh/pion/mediadevices/pull/468?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#diff-cGtnL2RyaXZlci9jbWRzb3VyY2UvY21kc291cmNlLmdv) | `3.33% <3.33%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion)

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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