lib/fetcher: Allow clients to append to User-Agent

We do already have `http-headers`, which potentially could be used to
allow clients to completely override the field, but it seems like the
more common use case is simply to append.

Closes: #1496
Approved by: cgwalters

src/libostree/ostree-fetcher.h

@@ -114,6 +114,9 @@ void _ostree_fetcher_set_tls_database (OstreeFetcher *self,
 void _ostree_fetcher_set_extra_headers (OstreeFetcher *self,
                                         GVariant      *extra_headers);
 
+void _ostree_fetcher_set_extra_user_agent (OstreeFetcher *self,
+                                           const char    *extra_user_agent);
+
 guint64 _ostree_fetcher_bytes_transferred (OstreeFetcher       *self);
 
 void _ostree_fetcher_request_to_tmpfile (OstreeFetcher         *self,