Unify the get_layers calls across all implements of manifest schemas to ensure we have a common type returned

Also renames some methods to make it more clear what kind of information they return

image/docker/schema2/list.py

@@ -211,8 +211,9 @@ class DockerSchema2ManifestList(ManifestInterface):
   def bytes(self):
     return self._manifest_bytes
 
-  @property
-  def layers(self):
+  def get_layers(self, content_retriever):
+    """ Returns the layers of this manifest, from base to leaf or None if this kind of manifest
+        does not support layers. """
     return None
 
   @property