The storage service should support using checksums that aren't SHA-256

[alexwlchan]

Quoting the BagIt RFC:

Starting with BagIt 1.0, bag creation and validation tools MUST
support the SHA-256 and SHA-512 algorithms [RFC6234] and SHOULD
enable SHA-512 by default when creating new bags. For backwards
compatibility, implementers SHOULD support MD5 [RFC1321] and SHA-1
[RFC3174]. Implementers are encouraged to simplify the process of
adding additional manifests using new algorithms to streamline the
process of in-place upgrades.

Right now we only support SHA-256. We should add support for other checksum algorithms.

[alexwlchan]

Failure modes we need to consider:

• There's no SHA-256/SHA-512 manifest
• The manifests have different entries
• One manifest is valid but the other isn't
• We have inconsistent manifests, e.g. a MD5 manifest and a SHA256 tag manifest