lib/deltas: Add signature check API for static-delta superblock

This retrieves the signatures and pass the static delta block as an array
of bytes to ostree_sign_data_verify().

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>

src/libostree/ostree-repo.h

@@ -32,6 +32,7 @@
 #include "ostree-repo-finder.h"
 #include "ostree-sepolicy.h"
 #include "ostree-gpg-verify-result.h"
+#include "ostree-sign.h"
 
 G_BEGIN_DECLS
 
@@ -1074,6 +1075,13 @@ gboolean ostree_repo_static_delta_execute_offline (OstreeRepo
                                                    GCancellable                  *cancellable,
                                                    GError                      **error);
 
+_OSTREE_PUBLIC
+gboolean ostree_repo_static_delta_verify_signature (OstreeRepo       *self,
+                                                    const char       *delta_id,
+                                                    OstreeSign       *sign,
+                                                    char            **out_success_message,
+                                                    GError          **error);
+
 _OSTREE_PUBLIC
 GHashTable *ostree_repo_traverse_new_reachable (void);