Digest subcommand

[puerco]

Summary

This PR adds a digest subcommand to the model_signing CLi. This allows other tools to find out the computed digest of a serialized model directory. It uses the same serializer as the sign command and supports adding files to the ignore list.

[puerco@babieco model-transparency] on  digest-subcommand 🐍 [venv]❯ python -m model_signing digest --help
Usage: python -m model_signing digest [OPTIONS] MODEL_PATH

  Computes the digest of a model.

  The digest subcommand serializes a model directory and computes the "root"
  digest (hash), the same used when signing and as the attestation subject.

  By default, git-related files are ignored (same behavior as the sign
  command). Use --no-ignore-git-paths to include them. To ignore other files
  from the directory serialization, use --ignore-paths.

Options:
  --ignore-paths IGNORE_PATHS     File paths to ignore when signing or
                                  verifying.
  --ignore-git-paths / --no-ignore-git-paths
                                  Ignore git-related files when signing or
                                  verifying.  [default: ignore-git-paths]
  --allow_symlinks                Whether to allow following symlinks when
                                  signing or verifying files.
  -h, --help                      Show this message and exit.

Checklist

• All commits are signed-off, using DCO
• All new code has docstrings and type annotations
• All new code is covered by tests. Aim for at least 90% coverage. CI is configured to highlight lines not covered by tests.
• Public facing changes are paired with documentation changes
• Release note has been added to CHANGELOG.md if needed

Closes #565

[mihaimaruseac]

Overall this looks good, but I have 2 points that I'd like to discuss a little bit more:

• First, should we only output just the root hash or all the hashes in the manifest?
• Second, maybe rather than importing from the private API, we could make the hashing config provide the root hash itself? Let me think a little bit more about this, while we discuss the first one.