lib: Add an API to GPG verify a commit given a remote

Conceptually we've been moving towards having our GPG verification
paths be per-remote.  The code internally supports this, but we
didn't expose an API to use it conveniently.

This came up when trying to add a new `gpgkeypath` option, since
right now rpm-ostree manually finds keyrings for the remote, and
hence it wasn't looking at the keypath, and said "Unknown key"
in status.

Adding an API fixes this nicely.

Closes: #576
Approved by: giuseppe

src/libostree/ostree-repo.h

@@ -1064,6 +1064,14 @@ OstreeGpgVerifyResult * ostree_repo_verify_commit_ext (OstreeRepo    *self,
                                                        GCancellable  *cancellable,
                                                        GError       **error);
 
+_OSTREE_PUBLIC
+OstreeGpgVerifyResult *
+ostree_repo_verify_commit_for_remote (OstreeRepo    *self,
+                                      const gchar   *commit_checksum,
+                                      const gchar   *remote_name,
+                                      GCancellable  *cancellable,
+                                      GError       **error);
+
 _OSTREE_PUBLIC
 OstreeGpgVerifyResult * ostree_repo_gpg_verify_data (OstreeRepo    *self,
                                                      const gchar   *remote_name,