Add filter to wp_update_plugins() to allow third-party updaters

[rmccue]

At the moment, most third party updaters use pre_set_site_transient_update_plugins to add extra update data. Unfortunately this method is prone to being a bit heavy handed, as update_plugins is updated in other places, such as when it has the expiration bumped just before the actual request. Custom updaters can't really hook into this correctly without doing some stupid tricks.

Attached patch adds core_version_check_response, plugins_update_check_response, and themes_update_check_response filters (not sure why you'd use the core one, but it's there for consistency).

[rmccue]

Add core_version_check_response, plugins_update_check_response, themes_update_check_response filters

[johnbillion]

It'd be nice to have a pre_ filter as well. Third-party updaters currently have to hook intohttp_request_args and check the request URL in order to filter out the plugins/themes that they don't want to send to api.wordpress.org for update checking.

[dd32]

Duplicate of #22377.