Migrate serialize_v0 to new API.

This is the middle layer of the API design work (sigstore#172). We add a manifest abstract class to represent various manifests (sigstore#111 sigstore#112) and also ways to serialize a model directory into manifests and ways to verify the manifests.

For now, this only does what was formerly known as `serialize_v0`. The v1 and the manifest versions will come soon.

Note: This has a lot of inspiration from sigstore#112, but makes the API work with all the usecases we need to consider right now.

Signed-off-by: Mihai Maruseac <mihaimaruseac@google.com>

model_signing/manifest/__init__.py

@@ -0,0 +1,13 @@
+# Copyright 2024 The Sigstore Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.

model_signing/manifest/manifest.py

@@ -0,0 +1,39 @@
+# Copyright 2024 The Sigstore Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Machinery for representing a serialized representation of an ML model.
+
+Currently, we only support a manifest that wraps around a digest. But, to
+support incremental updates and partial signature verification, we need a
+manifest that lists files and their digests. That will come in a future change,
+soon.
+"""
+
+from abc import ABCMeta
+from dataclasses import dataclass
+
+from model_signing.hashing import hashing
+
+
+class Manifest(metaclass=ABCMeta):
+    """Generic manifest file to represent a model."""
+
+    pass
+
+
+@dataclass
+class DigestManifest(Manifest):
+    """A manifest that is just a hash."""
+
+    digest: hashing.Digest