Added --downgrade option to otiotool.

[jminor]

This PR adds a --downgrade option to otiotool to make it easy to write out an older OTIO schema.

From the usage statement:

  --downgrade FAMILY:VERSION
                        Downgrade OTIO schema. Only relevant when --output is used to output an OTIO file. FAMILY:VERSION specifies which
                        schema family and version to use. If FAMILY: is omitted, the default OTIO_CORE: is used. For example `--downgrade
                        OTIO_CORE:0.14.0` is equivalent to `--downgrade 0.14.0`. See the OpenTimelineIO documentation for
                        OTIO_DEFAULT_TARGET_VERSION_FAMILY_LABEL for details.

Example:

otiotool -i new_file.otio --downgrade 0.14.0 -o old_file.otio

It uses the downgrade mechanism documented here.

[ssteinbach]

Could we get an option to dump the known families/versions? I think the API is there to query that.

[ssteinbach]

(apologies for the close/open, misclick)

[codecov-commenter]

Codecov Report

Merging #1485 (907480d) into main (3231f3c) will decrease coverage by 0.07%.
The diff coverage is 15.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1485      +/-   ##
==========================================
- Coverage   80.11%   80.05%   -0.07%     
==========================================
  Files         197      197              
  Lines       21699    21718      +19     
  Branches     4299     4307       +8     
==========================================
+ Hits        17385    17386       +1     
- Misses       2170     2182      +12     
- Partials     2144     2150       +6     

Flag	Coverage Δ
py-unittests	80.05% <15.00%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
...-opentimelineio/opentimelineio/console/otiotool.py	69.85% <15.00%> (-3.46%)	⬇️

... and 1 file with indirect coverage changes

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[meshula]

lgtm.

[jminor]

@ssteinbach I added --list-versions which prints this:

❯ otiotool  --list-versions
Available versions for --downgrade FAMILY:VERSION
  OTIO_CORE:0.14.0
  OTIO_CORE:0.15.0
  OTIO_CORE:0.16.0.dev1

[jminor]

Thanks for the feedback @JeanChristopheMorinPerso. I think I addressed all of your comments.