repo: Add API to load any object as a stream

We have APIs to load metadata as variants, and files as parsed
content/info/xattrs, but for some cases such as static deltas, all we
want is to operate on all objects in their canonical representation.

https://bugzilla.gnome.org/show_bug.cgi?id=706031

src/libostree/ostree-repo.h

@@ -198,6 +198,14 @@ gboolean ostree_repo_load_file (OstreeRepo         *self,
                                 GCancellable       *cancellable,
                                 GError            **error);
 
+gboolean ostree_repo_load_object_stream (OstreeRepo         *self,
+                                         OstreeObjectType    objtype,
+                                         const char         *checksum,
+                                         GInputStream      **out_input,
+                                         guint64            *out_size,
+                                         GCancellable       *cancellable,
+                                         GError            **error);
+
 gboolean      ostree_repo_query_object_storage_size (OstreeRepo           *self,
                                                      OstreeObjectType      objtype,
                                                      const char           *sha256,