zarr_content_read: include sizes for the "files" (and possibly "directories")

motivation: https://github.com/dandi/dandi-cli/pull/923#issuecomment-1050296836 -- in dandi-cli, to possibly avoid relatively expensive checksumming of the content to decide either a (changed) file within zarr needs to be reuploaded, we first check the size(s) of the file. If that one changed -- we know that we need to reupload even without computing its checksum.

It would be helpful if that API endpoint also returned sizes of the files (if it can do it without extra runtime penalty)

Start by locating the zarr_content_read API endpoint and its existing response tests, then inspect how it obtains Zarr file metadata and whether size lookup adds runtime cost. Done means the endpoint returns file sizes and, if supported without extra penalty, directory sizes so dandi-cli can compare content before checksumming.

Written by the indexing model from the issue text.