[feature] Support directory hashes

[laurentsimon]

As part of the effort to bring SLSA to ML https://github.com/google/model-transparency, we need to be able to sign directories. This requires the definition of a new "hash", i.e. how to serialize a directory. We have a PoC for this in the repo linked above, and need to support it in this repo too.

See https://github.com/slsa-framework/slsa-verifier/issues/730