feat: add VideoAudioDescription model and API for video audio description

[djoseph-apphelix]

Description

Add VideoAudioDescription model to store one audio description file per video, following the same pattern as VideoTranscript

Jira ticket

TNL2-577

[codecov]

Codecov Report

❌ Patch coverage is 53.33333% with 56 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.42%. Comparing base (9145aa6) to head (738f723).

Files with missing lines	Patch %	Lines
edxval/models.py	44.00%	28 Missing ⚠️
edxval/api.py	17.24%	24 Missing ⚠️
edxval/serializers.py	81.81%	2 Missing ⚠️
edxval/admin.py	90.90%	1 Missing ⚠️
edxval/utils.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #601      +/-   ##
==========================================
- Coverage   94.86%   93.42%   -1.45%     
==========================================
  Files          33       34       +1     
  Lines        3350     3468     +118     
  Branches      129      137       +8     
==========================================
+ Hits         3178     3240      +62     
- Misses        154      210      +56     
  Partials       18       18              

Flag	Coverage Δ
unittests	93.42% <53.33%> (-1.45%)	⬇️

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