Verification of unknown signature configurations

[spencerschrock]

Description

Under the (sigstore) API introduced in #323, there's sign and verify. The defaults work well together assuming the entity signing is the same entity verifying, and that the defaults haven't changed between sign/verify:

model_signing_api.sign(model_path, signature_path)
model_signing_api.verify(model_path, signature_path, identity=expected_identity)

However, verification of the underlying signature requires knowledge of the HashingConfig used during signing. Again, if everyone uses the default, or the signing/verifying entities are the same that's fine. But defaults can change over time, and one model you download might be signed differently than another.

For the most flexible verification, API users need to inspect the signature before verifying it and:

1. Determine signature type (currently only sigstore DSSE envelope).
2. Determine manifest type, (all 4 combinations of { file, shard} x {manifest, digest of digests} )
   1. If using file shards, confirm shard size
3. Confirm hashing algorithm (sha or blake)

I don't believe any of these have any trust implications, so the default verify function could do a lot of this transparently to the user.