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[bot]]

Codecov Report

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

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

```diff @@ 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](https://codecov.io/gh/pion/mediadevices/pull/465?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | Coverage Δ | | |---|---|---| | [pkg/codec/x264/x264.go](https://codecov.io/gh/pion/mediadevices/pull/465?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#diff-cGtnL2NvZGVjL3gyNjQveDI2NC5nbw==) | `60.41% <0.00%> (-4.70%)` | :arrow_down: | | [pkg/driver/cmdsource/audiocmd.go](https://codecov.io/gh/pion/mediadevices/pull/465?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/465?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/465?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/465?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)` | | | [track.go](https://codecov.io/gh/pion/mediadevices/pull/465?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#diff-dHJhY2suZ28=) | `25.44% <33.33%> (+0.13%)` | :arrow_up: | | [pkg/codec/vpx/vpx.go](https://codecov.io/gh/pion/mediadevices/pull/465?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#diff-cGtnL2NvZGVjL3ZweC92cHguZ28=) | `81.00% <65.00%> (-2.52%)` | :arrow_down: | 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]

Thanks! Will do.