pull: Add support for http-headers option

Some deployments may want to gate access to content based on things
like OAuth.  In this model, the client system would normally compute a
token and pass it to the server via an API.

We could theoretically support this in the remote config too, but
that'd be a bit weird for OAuth as the information is dynamic.
Therefore this cleans up the code a little bit to more clearly handle
the case that the fetcher is initialized from both remote config
data plus pull options.

Closes: #574
Approved by: giuseppe

src/libostree/ostree-fetcher.h

@@ -71,6 +71,9 @@ void _ostree_fetcher_set_client_cert (OstreeFetcher *fetcher,
 void _ostree_fetcher_set_tls_database (OstreeFetcher *self,
                                        GTlsDatabase *db);
 
+void _ostree_fetcher_set_extra_headers (OstreeFetcher *self,
+                                        GVariant      *extra_headers);
+
 guint64 _ostree_fetcher_bytes_transferred (OstreeFetcher       *self);
 
 void _ostree_fetcher_mirrored_request_with_partial_async (OstreeFetcher         *self,