Add H264 RTP depayloader

[brzep]

Depayloads H264 from RTP. Supports following NALU types: Single NALU, STAP-A, FU-A.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.97%. Comparing base (38e05e2) to head (be1ad65).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #224      +/-   ##
==========================================
+ Coverage   87.79%   87.97%   +0.18%     
==========================================
  Files          54       59       +5     
  Lines        2752     2803      +51     
==========================================
+ Hits         2416     2466      +50     
- Misses        336      337       +1     

Files with missing lines	Coverage Δ
lib/ex_webrtc/rtp/depayloader.ex	100.00% <100.00%> (ø)
lib/ex_webrtc/rtp/h264/depayloader.ex	100.00% <100.00%> (ø)
lib/ex_webrtc/rtp/h264/nal_formats/fu.ex	100.00% <100.00%> (ø)
lib/ex_webrtc/rtp/h264/nal_formats/fu/header.ex	100.00% <100.00%> (ø)
lib/ex_webrtc/rtp/h264/nal_formats/stap_a.ex	100.00% <100.00%> (ø)
lib/ex_webrtc/rtp/h264/nal_header.ex	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[sgfn]

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[sgfn]

It would be nice if we could add tests for the other modules (fu, fu/header, stap_a, nal_header), like in test/rtp/vp8/payload_test.exs or test/rtp/av1/ as well. But I won't push for this, as we have other things on the roadmap -- if @mickel8 approves, you can merge 👍