lib/deltas: Check signed delta in execute_offline

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>