Add get_asset_metadata() function

[jwodder]

Closes #692.

[codecov]

Codecov Report

Merging #693 (0513032) into master (00357fa) will increase coverage by 0.21%.
The diff coverage is 90.18%.

@@            Coverage Diff             @@
##           master     #693      +/-   ##
==========================================
+ Coverage   83.82%   84.03%   +0.21%     
==========================================
  Files          62       59       -3     
  Lines        6595     5624     -971     
==========================================
- Hits         5528     4726     -802     
+ Misses       1067      898     -169     

Flag	Coverage Δ
unittests	84.03% <90.18%> (+0.21%)	⬆️

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

Impacted Files	Coverage Δ
dandi/__init__.py	70.00% <ø> (ø)
dandi/cli/cmd_digest.py	100.00% <ø> (ø)
dandi/cli/tests/test_download.py	100.00% <ø> (+36.36%)	⬆️
dandi/exceptions.py	100.00% <ø> (ø)
dandi/support/generatorify.py	0.00% <0.00%> (-64.37%)	⬇️
dandi/tests/test_organize.py	93.60% <ø> (ø)
dandi/cli/cmd_shell_completion.py	32.00% <32.00%> (ø)
dandi/validate.py	52.94% <42.85%> (-0.23%)	⬇️
dandi/cli/cmd_organize.py	76.87% <50.00%> (-0.43%)	⬇️
dandi/cli/cmd_delete.py	81.81% <66.66%> (+1.81%)	⬆️
... and 53 more

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[yarikoptic]

Looks good to me and even green, so let's proceed. Thank you @jwodder !