pull: Add summary-{,sig-}bytes options to ostree_repo_pull()

pull: Add summary-{,sig-}bytes options to ostree_repo_pull()

These allow the `summary` and `summary.sig` files to be cached at a
higher layer (for example, flatpak) between related pull operations (for
example, within a single flatpak transaction). This avoids
re-downloading `summary.sig` multiple times throughout a transaction,
which increases the transaction’s latency and introduces the possibility
for inconsistency between parts of the transaction if the server changes
its `summary` file part-way through.

In particular, this should speed up flatpak transactions on machines
with high latency network connections, where network round trips have a
high impact on the latency of an overall operation.

Signed-off-by: Philip Withnall <withnall@endlessm.com>