Extend avc1 to support multiple video sample description extensions

[finnhughes]

avc1 can support multiple extensions including avcC and pasp
Not sure where this is documented in ISO standards, I found it in an mp4 file and then found it documented for quicktime https://developer.apple.com/library/archive/documentation/QuickTime/QTFF/QTFFChap3/qtff3.html

avcC has optional fields at the end which are now ignored with GreedyBytes if present, without this tweak I had mp4 files I couldn't parse if they had any extra info after the pps.

I'm using the BytesIO so that I can assert the expected data was parsed.

Note this is a breaking change since avcC used to be stored in avc_data but is now stored in extensions[avcC]. In QT the order is not defined so I think the extensions list is the right way to handle it.

[codecov-io]

Codecov Report

Merging #6 into master will increase coverage by 1.39%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   83.11%   84.51%   +1.39%     
==========================================
  Files           3        3              
  Lines         154      155       +1     
==========================================
+ Hits          128      131       +3     
+ Misses         26       24       -2

Impacted Files	Coverage Δ
src/pymp4/parser.py	84% <100%> (+1.74%)	⬆️

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