fix(apk-polkit-rs): do not block on polkit request

Although the dbus_interface allows non-async methods, according to the docs that is only for convenience, and the methods are called from an async context. In consequence, doing blocking calls in check_polkit_auth is undefined behavior and crashes at runtime. The solution is to move the authentication and the related methods to async. Unfortunately, that requires a number of syntax changes which are a bit annoying to deal with.

This fixes !34 (comment 263746)