Playout delay support

[luc4s]

Hi,

First, thanks for the awesome library :)

I needed playout delay header extension support for a low latency streaming application (https://webrtc.googlesource.com/src/+/refs/heads/main/docs/native-code/rtp-hdrext/playout-delay/README.md), so I thought I could share it.

[jlaine]

This PR has no unit test?

[luc4s]

True, I haven't added any, I will add some tests.

[luc4s]

Ok so I added a test for the playout delay and adapted the ones for header extensions.

Also I was wondering: regarding setPlayoutDelay, is raising a ValueError when one of the delays is out of range ok for you?
I was also thinking of clamping them, but then I did not like having them 'silently' clamped.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (9f14474) 100.00% compared to head (13bf23d) 100.00%.

❗ Current head 13bf23d differs from pull request most recent head 2a5a978. Consider uploading reports for the commit 2a5a978 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #668   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           30        31    +1     
  Lines         5816      5801   -15     
=========================================
- Hits          5816      5801   -15     

Impacted Files	Coverage Δ
src/aiortc/codecs/__init__.py	100.00% <ø> (ø)
src/aiortc/rtcrtpsender.py	100.00% <100.00%> (ø)
src/aiortc/rtp.py	100.00% <100.00%> (ø)

... and 16 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
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[luc4s]

Is there a problem with the runners? Seems like the test never started:

2022-04-04T13:49:27.6313834Z Requested labels: macos-latest
2022-04-04T13:49:27.6313880Z Job defined at: aiortc/aiortc/.github/workflows/tests.yml@refs/pull/668/merge
2022-04-04T13:49:27.6313901Z Waiting for a runner to pick up this job...
2022-04-04T13:49:30.3209105Z Job is waiting for a hosted runner to come online.
2022-04-04T13:49:42.6601603Z Job is about to start running on the hosted runner: GitHub Actions 4 (hosted)

[DerGenaue]

@jlaine Is there a chance for this PR to progress some time soon?