Give RemoteAsset a created attribute

[jwodder]

Motivated by dandi/dandisets#84.

[codecov]

Codecov Report

Merging #781 (4eee910) into master (27dd3e7) will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #781      +/-   ##
==========================================
- Coverage   85.22%   85.11%   -0.11%     
==========================================
  Files          59       59              
  Lines        6039     6055      +16     
==========================================
+ Hits         5147     5154       +7     
- Misses        892      901       +9     

Flag	Coverage Δ
unittests	85.11% <100.00%> (-0.11%)	⬇️

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

Impacted Files	Coverage Δ
dandi/dandiapi.py	92.49% <100.00%> (-0.17%)	⬇️
dandi/tests/test_dandiapi.py	100.00% <100.00%> (ø)
dandi/support/generatorify.py	0.00% <0.00%> (-16.10%)	⬇️
dandi/dandiarchive.py	82.23% <0.00%> (-1.53%)	⬇️
dandi/tests/test_metadata.py	100.00% <0.00%> (ø)
dandi/cli/cmd_validate.py	85.00% <0.00%> (+0.38%)	⬆️
dandi/validate.py	53.48% <0.00%> (+0.54%)	⬆️
dandi/metadata.py	82.17% <0.00%> (+2.50%)	⬆️

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[yarikoptic]

ok, lets proceed and release this. Thank you @jwodder