This should be a property directly on the result, not in its payload, since the payload is specific to each kind of result.

Bug 1578583 implemented result span as a UrlbarUtils function that always returns 3 for tip results. Any other new types of results we add in the future will likely follow the same approach -- i.e., types will be implemented and supported directly in mozilla-central with result spans returned by UrlbarUtils. We don't need the webext API to support result span after all.