Allow dictionary representation of `Dandiset` to have extra attributes #1314

Job	Run time
test-dandi-cli (windows-2019, 3.8, master, normal)	4m 31s
test-dandi-cli (windows-2019, 3.8, release, normal)	2m 54s
test-dandi-cli (ubuntu-latest, 3.8, master, normal)	11m 21s
test-dandi-cli (ubuntu-latest, 3.8, release, normal)	1m 31s
test-dandi-cli (macos-latest, 3.8, master, normal)	3m 18s
test-dandi-cli (macos-latest, 3.8, release, normal)	2m 4s
test-dandi-cli (ubuntu-latest, 3.8, dandi-devel, master)	11m 7s
test-dandi-cli (ubuntu-latest, 3.8, dandi-devel, release)	1m 25s
	38m 11s

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