Add a filter on the Plugin and Theme installers table listing API args, this allows for plugins to hook in and add custom args to plugins_api() and themes_api() for the Plugin/Theme listing tabls. Fixes #18169

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git-svn-id: https://core.svn.wordpress.org/trunk@25055 1a063a9b-81f0-0310-95a4-ce76da25c4cd