lib/deltas: Check signed delta in execute_offline

Add a new function `ostree_repo_static_delta_execute_offline_with_signature`
which takes a signature engine to verify the delta before applying it.
The `ostree_repo_static_delta_execute_offline` is just a wrapper to this
new function, passing a NULL signature engine.
When this function is called without signature engine, but with a sign
delta, it will only fails if `sign-verify-deltas` is set to true in repo
core options.

This commits move signature existence check and delta signature
verification to share common parts between existing APIs and the new
function.

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

src/libostree/ostree-repo.h

@@ -1068,6 +1068,14 @@ gboolean ostree_repo_static_delta_generate (OstreeRepo                   *self,
                                             GCancellable                 *cancellable,
                                             GError                      **error);
 
+_OSTREE_PUBLIC
+gboolean ostree_repo_static_delta_execute_offline_with_signature (OstreeRepo   *self,
+                                                                  GFile        *dir_or_file,
+                                                                  OstreeSign   *sign,
+                                                                  gboolean     skip_validation,
+                                                                  GCancellable *cancellable,
+                                                                  GError       **error);
+
 _OSTREE_PUBLIC
 gboolean ostree_repo_static_delta_execute_offline (OstreeRepo                    *self,
                                                    GFile                         *dir_or_file,