Allow plugins to force an update of an UpdateSite

Currently plugins can request an asynchronous update of an `UpdateSite` but that request may or may not happen.
the reasons it may not happen is that the UpdateSite was updated recently (and cached) however a plugin should still be able to force the update if it knows reasons why an update should be available.

removing the cache and then asking for an asynchronous update is a hacky solution - (especially if there is a transient error fetching the json) - so this request is to unrestrict updateDirectlyNow()