reconfirm with user if really wanting to upload > 10k files

inspired by https://github.com/dandi/dandi-cli/pull/1077

ATM, some new .zarr-based/like directory format could be used by a user, but with us not detecting as some kind of "zarr file" to become a single asset, and thus causing `dandi upload` uploading potentially 100s of thousands of new tiny assets. I wonder if we should prevent that for everyone's sake. E.g. we could, if we detect that e.g. over 10k files to be uploaded, ask user to reconfirm if that is an intended action. WDYT @satra ?