Fixes #8514 - Add support for requests to smart proxy discovery plugin

For myself, I agree about not wanting to re-introduce deface to the plugin. However, we have little choice - the 'discovery' feature will be present on the proxy, and Foreman needs to know how to handle that. I see that foreman-salt had to do a similar thing in adding the salt_proxy_id to the host and hostgroup forms. As such, I don't think we can get around this :)