ostree-repo: Add a new ostree_repo_verify_local_summary() API

ostree-repo: Add a new ostree_repo_verify_local_summary() API

This will be used in the following commit to add signature verification
to `ostree summary` when used to view the summary of a local repository.

It needs to be separate from `ostree_repo_verify_summary()`, as that API
expects to take a remote name and use the keyring for that remote. As
far as I can tell there isn’t a way to use an arbitrary GPG homedir to
verify a summary file, as is needed when running on the OSTree server
rather than on a client checkout.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>