Site Health: Add auto-update information to for plugins and themes.

New filters:

* `plugin_auto_update_debug_str`
* `theme_auto_update_debug_str`

Fixes #50663.
Props pbiron, audrasjb, davidbaumwald.

Built from https://develop.svn.wordpress.org/trunk@48546


git-svn-id: https://core.svn.wordpress.org/trunk@48308 1a063a9b-81f0-0310-95a4-ce76da25c4cd