Add --preserve-tree option to dandi download

[jwodder]

Closes #1460.

[codecov]

Codecov Report

Attention: Patch coverage is 97.05882% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.60%. Comparing base (c0b64d5) to head (c583a89).

Files	Patch %	Lines
dandi/dandiarchive.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1467      +/-   ##
==========================================
+ Coverage   88.55%   88.60%   +0.05%     
==========================================
  Files          77       77              
  Lines       10535    10556      +21     
==========================================
+ Hits         9329     9353      +24     
+ Misses       1206     1203       -3     

Flag	Coverage Δ
unittests	88.60% <97.05%> (+0.05%)	⬆️

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[kabilar]

Thank you @jwodder. This feature will be very helpful.

[yarikoptic]

Thank you!

[github-actions]

🚀 PR was released in 0.63.0 🚀