Allow plugins to automatically add pull-through content to repositories

[gerrod3]

Currently when a plugin supports pull-through caching, using the provided pulpcore machinery, the content that is cached in a pull-through request is created as an orphan and has to be manually added to a repository by the user (the admin user since only admins can view content not in a repository!) We should allow plugins to dispatch a task to add the pull-through content to an associated repository attached to the distribution.