Let users define GlobalPolicy and NilPolicy class

In https://gitlab.com/alexkalderimis/declarative-policy/-/blob/4f0d1ab3598c9bd8f3debd56224a8f0941f7f7c5/lib/declarative_policy.rb#L39-40 we use hardcoded class names such as GlobalPolicy and NilPolicy.

While this works for GitLab it'd be great to let users define/inject those special classes.