REST API: add WP_REST_Posts_Controller route to fetch page counts

[ramonopoly]

The PR proposes to extend WP_REST_Posts_Controller with a new route /count to fetch page counts, e.g., wp/v2/pages/count.

The total number of post counts is fetched by using ​wp_count_posts.

​https://github.com/WordPress/gutenberg/pull/67719

There is a need for a performant post counts check in the block editor. See: ​https://github.com/WordPress/gutenberg/pull/65223#issuecomment-2381323815

Contrast this feature with $query->found_posts which returns the total count for a specific query only and would degrade performance given n posts.

## What?

Syncs ​https://github.com/WordPress/gutenberg/pull/66294

The PR proposes a new endpoint to fetch the total number of post counts by post status using ​wp_count_posts.

## Why?

The need for a performant post counts check for the block editor. See: ​https://github.com/WordPress/gutenberg/pull/65223#issuecomment-2381323815

Contrast this with $query->found_posts which returns the total count for a specific query only and would degrade performance given n posts.

## How?

Following the simplicity of ​wp/v2/statuses, which wraps ​get_post_stati, creating a basic route to return post type counts, e.g., wp/v2/counts/<post_type>

## Testing Instructions

Run the tests: npm run test:php -- --filter=WP_Test_REST_Post_Counts_Controller

Open up the site editor and have a play with the console:

Examples:

await wp.apiFetch( { path: '/wp/v2/counts/page' } )

/*
{
    "publish": 1,
    "future": 0,
    "draft": 2,
    "pending": 0,
    "private": 0,
    "trash": 0,
    "auto-draft": 0
}
*/

await wp.apiFetch( { path: '/wp/v2/counts/wp_block' } )

/*
{
    "publish": 2,
    "future": 0,
    "draft": 0,
    "pending": 0,
    "private": 0,
    "trash": 0,
    "auto-draft": 0
}
*/

Also add a custom post type and ensure it appears in the response:

function add_my_custom_post_type() {
        register_post_status(
                'sweaty-palms',
                array(
                        'label'                     => _x( 'Sweaty palms', 'post' ),
                        'label_count'               => _n_noop(
                                'Sweaty palms <span class="count">(%s)</span>',
                                'Sweaty palms <span 
class="count">(%s)</span>'
                        ),
                        'public'                    => true,
                        'exclude_from_search'       => false,
                        'show_in_admin_all_list'    => true,
                        'show_in_admin_status_list' => true,
                )
        );
}
add_action( 'init', add_my_custom_post_type );

Trac ticket: https://core.trac.wordpress.org/ticket/62390

I appreciate the quick review and suggestions.

This new endpoint needs tests.

I am aware. had already written a prominent TODO in the description to add them.

Thanks for helping me think this over @spacedmonkey

Given everything I've tested I have a mind to revert back to ​https://github.com/WordPress/wordpress-develop/blob/95db722a2f73fd238312ed026f442c88363b3cd6/src/wp-includes/rest-api/endpoints/class-wp-rest-post-counts-controller.php, which introduced an embeddable, separate endpoint with a dedicated schema.

So far the only arguments to extend WP_REST_Posts_Controller has been because it seems like the natural thing to do.

Is it worth trying add a custom schema for the /counts route?