REST API: Add accessor functions for post_status and post_parent

[rmccue]

In order to enable better permission checks for Customiser Changesets, these need to be filterable.

See ​xwp/wp-customize-snapshots#32. Split from #38701.

[westonruter]

The first thing to do is to replace any current usages of $post->post_status with simply get_post_status( $post->ID ).

See ​https://github.com/xwp/wordpress-develop/pull/214

The lesser-important change to core is to handle permission checks for a post/page that has an inherit status when the post_parent itself is modified in the customized state. In this case, there needs to be filters applied on the post_parent value itself. However, there is no get_post_parent() function with any filters currently in core.

[kadamwhite]

@westonruter is this still a desired enhancement? We're turning over some long-forgotten bugs in triage today :)

[westonruter]

This was needed for the Customize Posts plugin to be able to add filters to override a post field based on what has been modified in the Customizer changeset, but this plugin is not being maintained currently so it is not an active need.