Centralize walking of the zarr folders for checksum/upload purposes

[yarikoptic]

Inspired by revelations etc in

• Adopt dandischema.digests.zarr.get_checksum() to dandi-cli: use zarr_checksum library constructs instead of copies in dandischema #1371

ideally should be the same walker to be used by dandi-cli and anything else (e.g. dandi-archive) which is interested in computing the checksum over zarr

Related

• Generalize zarr_checksum into folder_checksum with a set of format-specific (e.g. zarr) adapters? zarr_checksum#50
  which suggests a somewhat larger scoped generalization of the zarr_checksum.