Added otiotool --remove-metadata-key option

[jminor]

This PR adds a new --remove-metadata-key feature to otiotool. If specified, this option looks for the given metadata key(s) on all objects in the OTIO and removes it when found. This could be used to remove private, sensitive, or extraneous metadata from an OTIO.

  --remove-metadata-key KEY(s) [KEY(s) ...]
                        Remove one or more metadata keys from all objects.

This PR also improves the otiotool usage docs, adding some examples, clarifying some arguments, and making the short summary more readable by using the --long-options instead of single-letter options.

usage: otiotool [-h] [--input PATHs) [PATH(s) ...]] [--video-only] [--audio-only] [--only-tracks-with-name NAME(s) [NAME(s) ...]]
                [--only-tracks-with-index INDEX(es) [INDEX(es) ...]] [--only-clips-with-name NAME(s) [NAME(s) ...]]
                [--only-clips-with-name-regex REGEX(es) [REGEX(es) ...]] [--remove-transitions] [--trim START END] [--flatten TYPE] [--keep-flattened-tracks]
                [--stack] [--concat] [--relink-by-name FOLDER(s) [FOLDER(s) ...]] [--copy-media-to-folder FOLDER] [--remove-metadata-key KEY(s) [KEY(s) ...]]
                [--redact] [--stats] [--list-clips] [--list-tracks] [--list-media] [--verify-media] [--list-markers] [--inspect NAME(s) [NAME(s ...]]
                [--downgrade FAMILY:VERSION] [--list-versions] [--output PATH]

[codecov-commenter]

Codecov Report

Merging #1642 (6c65bf2) into main (eaf8569) will decrease coverage by 0.08%.
The diff coverage is 9.09%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1642      +/-   ##
==========================================
- Coverage   79.91%   79.83%   -0.08%     
==========================================
  Files         197      197              
  Lines       21731    21753      +22     
  Branches     4339     4350      +11     
==========================================
+ Hits        17366    17367       +1     
- Misses       2213     2233      +20     
- Partials     2152     2153       +1     

Flag	Coverage Δ
py-unittests	79.83% <9.09%> (-0.08%)	⬇️

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Files Changed	Coverage Δ
...-opentimelineio/opentimelineio/console/otiotool.py	66.21% <9.09%> (-3.65%)	⬇️

... and 1 file with indirect coverage changes

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[apetrynet]

If I read this correctly it will only remove a key from the "top level" of a metadata dict.
I believe this will suffice in most cases, but we should perhaps mention this in the docs?

[jminor]

@apetrynet I addressed your note about "top level" in the usage statement. Does that look good to you?

[apetrynet]

Looks good to me @jminor
I also like the more verbose help strings in the other arguments.