Add optional comment field to Marker

[nickblt]

Link the Issue(s) this Pull Request is related to.

Fixes #744

Summarize your change.

This adds an optional comment field to the Marker object as requested in #744

Reference associated tests.

Added new tests that specifically test to make sure the comment is being added and serialized/deserialized successfully

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: nickblt / name: Nicholas Bui LeTourneau (4eeed2f, 09dd254, 74dd686)

[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (d22935e) 79.84% compared to head (74dd686) 79.83%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1698      +/-   ##
==========================================
- Coverage   79.84%   79.83%   -0.01%     
==========================================
  Files         197      197              
  Lines       21796    21814      +18     
  Branches     4358     4362       +4     
==========================================
+ Hits        17403    17416      +13     
- Misses       2232     2233       +1     
- Partials     2161     2165       +4     

Flag	Coverage Δ
py-unittests	79.83% <59.09%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
tests/test_marker.py	95.55% <100.00%> (+1.26%)	⬆️
src/opentimelineio/marker.h	87.50% <50.00%> (-12.50%)	⬇️
src/opentimelineio/marker.cpp	57.89% <25.00%> (-4.61%)	⬇️
...entimelineio-bindings/otio_serializableObjects.cpp	36.07% <16.66%> (-0.02%)	⬇️

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[meshula]

Looks good to me.