Meta Boxes: Add the `block_editor_meta_box_hidden_fields` action.

Lacking an appropriate action in the classic editor, plugins that add meta boxes have historically hooked into various actions in order to add hidden input fields.

This change also adds backwards compatibility for two of the most common: `edit_form_after_title`, and `edit_form_advanced`.

Props pento, danielbachhuber.
Merges [43882] to trunk.
Fixes #45283.
Built from https://develop.svn.wordpress.org/trunk@44241


git-svn-id: https://core.svn.wordpress.org/trunk@44071 1a063a9b-81f0-0310-95a4-ce76da25c4cd