The previous filter failed the goal of allowing developers to filter all providers before they are registered, since it only filtered the built-in ones.
The more specific `wp_sitemaps_add_provider` filter enables exactly that, as it filters every sitemap provider right before it is added to the sitemaps registry.
Props pbiron, pfefferle, Chouby, swissspidy.
Fixes#50660.
Built from https://develop.svn.wordpress.org/trunk@48543
git-svn-id: http://core.svn.wordpress.org/trunk@48305 1a063a9b-81f0-0310-95a4-ce76da25c4cd