Block Editor: Preload `wp/v2/media` with OPTIONS for caps check.

Also introduces a `block_editor_preload_paths` filter for plugins and themes to preload additional data.

Merges [43833] from the 5.0 branch to trunk.

Props imath, mattheu, danielbachhuber.
Fixes #45194.


Built from https://develop.svn.wordpress.org/trunk@44172


git-svn-id: https://core.svn.wordpress.org/trunk@44002 1a063a9b-81f0-0310-95a4-ce76da25c4cd