Sitemaps: Replace `wp_sitemaps_register_providers` filter with more suitable...

Sitemaps: Replace `wp_sitemaps_register_providers` filter with more suitable `wp_sitemaps_add_provider` filter.

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: https://core.svn.wordpress.org/trunk@48305 1a063a9b-81f0-0310-95a4-ce76da25c4cd