core: Add a function creating an archive-z2 content stream

It is quite similar to the already existing
ostree_raw_file_to_content_stream function, so I factored the common
part to a separate function. The difference is that we cannot report
the size of the resulting stream.

Can be useful for serving a "bare" repository as a faked "archive-z2"
repository.

Closes: #308
Approved by: cgwalters

src/libostree/libostree.sym

@@ -341,5 +341,5 @@ global:
 LIBOSTREE_2016.6 {
 global:
         ostree_repo_remote_fetch_summary_with_options;
-
+        ostree_raw_file_to_archive_z2_stream;
 } LIBOSTREE_2016.5;