md5 / sha256 should be optional

[kulnor]

The following error is being reported for non-live datasets by the Python package:

 [FileObject(ACSPUMS1Y2023.json)] At least one of these properties should be defined: ['md5', 'sha256'].

These hash values are very rarely available in public data catalogs and other repositories and should be optional (or recommended).
It would also not be practical to compute this when producing Croissant as we do not have access to or have a local copy of the file.

Note that:

• md5 is not mentioned in the specifications at https://docs.mlcommons.org/croissant/docs/croissant-spec.html#fileobject
• other algorithms could be used as well, such as sha1. A controlled vocabulary + value pair may be more practical than predefined properties.