Make use of /assets/{asset_id}/info/ endpoint

[jwodder]

This PR makes use of the /assets/{asset_id}/info/ endpoint added by dandi/dandi-archive#681 to construct BaseRemoteAssets that have all of their "meta-metadata" from their Django serializations, thereby accomplishing the following:

• Add created and modified fields to BaseRemoteAsset
• Add BaseRemoteBlobAsset and BaseRemoteZarrAsset subclasses of BaseRemoteAsset with blob and zarr attributes, respectively
  • These subclasses determine the return value of asset_type based on overloading rather than relying on encodingFormat
  • filetree and iterfiles() have been moved from RemoteZarrAsset to BaseRemoteZarrAsset
• BaseRemoteAsset.from_metadata() has been replaced with BaseRemoteAsset.from_base_data()
• Support download of Zarrs identified by https://<server>/api/assets/<asset-id>/ URLs (Closes Support downloading Zarrs identified by https://<server>/api/assets/<asset-id>/ URLs #883)

[codecov]

Codecov Report

Merging #895 (bc29b0c) into master (8d14434) will increase coverage by 0.03%.
The diff coverage is 90.56%.

@@            Coverage Diff             @@
##           master     #895      +/-   ##
==========================================
+ Coverage   86.66%   86.70%   +0.03%     
==========================================
  Files          61       61              
  Lines        7268     7282      +14     
==========================================
+ Hits         6299     6314      +15     
+ Misses        969      968       -1     

Flag	Coverage Δ
unittests	86.70% <90.56%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
dandi/download.py	86.96% <75.00%> (-0.24%)	⬇️
dandi/dandiapi.py	88.39% <90.90%> (-0.23%)	⬇️
dandi/tests/test_download.py	100.00% <100.00%> (ø)
dandi/dandiarchive.py	84.42% <0.00%> (+2.04%)	⬆️

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[yarikoptic]

Looks LGTM upon cursory review, no tests had to be adjusted (good), and test for new functionality added (good). Thank you @jwodder !