pull: Allow additional HTTP headers for summary fetch
pull: Allow additional HTTP headers for summary fetch

Read the http-headers (a(ss)) option in
ostree_repo_remote_fetch_summary_with_options like
ostree_repo_pull_with_options and add the headers to the fetcher. This
allows things like providing additional authorization headers to the
HTTP requests.

Closes: #839
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree