Add download support for URLs "seen" on dandidav instance

ATM dandidav instance (webdav.dandiarchive.org) is THE ONLY convenient to users way to navigate the zarrs in the archive , so if we add support for downloading those urls, we provide user convenience to download them

- https://webdav.dandiarchive.org/dandisets/000108/draft/sub-MITU01/ses-20210521h17m17s06/micr/sub-MITU01_ses-20210521h17m17s06_sample-178_stain-LEC_run-1_chunk-1_SPIM.ome.zarr/1/0/

tricky part is that for individual files we have s3 urls directly:

- https://dandiarchive.s3.amazonaws.com/zarr/56509720-870c-4f43-ae41-7b75f9590722/1/.zattrs

if first one is easy, 2nd - not sure.

Those could simply be dumped into the name corresponding to the end of the URL + respecting `-o`.