Remove code for calculating Zarr checksums

[jwodder]

Now that we have zarr_checksum, having a separate module for checksumming Zarrs in dandischema is redundant.

• Replace all uses of dandischema.digests.zarr with zarr_checksum
  • dandi/dandi-cli: dandi/files/zarr.py
    • Adopt dandischema.digests.zarr.get_checksum() to dandi-cli: use zarr_checksum library constructs instead of copies in dandischema dandi-cli#1371
  • dandi/dandi-archive: dandiapi/zarr/tests/test_zarr.py
    • test_zarr.py: Import EMPTY_CHECKSUM from zarr_checksum dandi-archive#1775
  • dandischema: dandischema/models.py
    • Remove Zarr checksum code #217
  • Any others?
• Delete dandischema.digests.zarr and its test code
  • Remove Zarr checksum code #217
  • This change should be released in a minor version update.

[yarikoptic]

NB: asking in https://github.com/dandi/dandi-cli/pull/1371/files#r1420705289 on either we could/should reuse a higher level constructs in zarr_checksum

[jwodder]

Removal of the Zarr code from dandischema will require rewriting the code for extracting total size from a Zarr checksum. This will mean adding zarr_checksum as a dependency to dandischema, but:

• zarr_checksum currently requires Pydantic v2, which dandischema does not yet support
  • Alternative: Remove Pydantic dependency zarr_checksum#53
• zarr_checksum depends on two heavy libraries that could easily be removed from the default dependencies but haven't yet: Make boto3 and zarr extra dependencies zarr_checksum#39