Expose reosurce descriptors from manifests

We aim this to be similar to in-toto's `ResourceDescriptor`. To support cases where in-toto cannot be directly used, we make this a dataclass that can be mapped to in-toto when needed, and used as its own otherwise.

Not all fields from in-toto are specified at this moment. All fields here must be present, unlike in-toto, where all are optional.

See https://github.com/in-toto/attestation/blob/main/spec/v1/resource_descriptor.md for the in-toto specification.

This is the first separable PR for the signing support (see full draft on #253)

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

model_signing/test_support.py

@@ -43,6 +43,16 @@
 ]
 
 
+# All directory models to use in testing, where only non empty directory models
+# are supported. See also `all_test_models` comments.
+all_non_empty_directory_test_models = [
+    "sample_model_folder",
+    "deep_model_folder",
+    "model_folder_with_empty_file",
+    "symlink_model_folder",
+]
+
+
 def get_first_directory(path: pathlib.Path) -> pathlib.Path:
     """Returns the first directory that is a children of path.