Switch content retrieval in manifests to be behind an interface

This allows for easy separation of retrieval of config blobs vs manifests

image/docker/schemautil.py

@@ -0,0 +1,24 @@
+import json
+
+from image.docker.interfaces import ContentRetriever
+
+class ContentRetrieverForTesting(ContentRetriever):
+  def __init__(self, digests=None):
+    self.digests = digests or {}
+
+  def add_digest(self, digest, content):
+    self.digests[digest] = content
+
+  def get_manifest_bytes_with_digest(self, digest):
+    return self.digests.get(digest)
+
+  def get_blob_bytes_with_digest(self, digest):
+    return self.digests.get(digest)
+
+  @classmethod
+  def for_config(cls, config_obj, digest, size):
+    config_str = json.dumps(config_obj)
+    padded_string = config_str + ' ' * (size - len(config_str))
+    digests = {}
+    digests[digest] = padded_string
+    return ContentRetrieverForTesting(digests)