Add hook to WP_Sitemaps::render_sitemaps()

[Cybr]

Currently, we cannot tweak the sitemap's content parameters or headers.

To allow this without having to test whether the sitemap is outputting (there's currently no proper way, see #51543 and #56954), in WP_Sitemaps::render_sitemaps(), after the sitemaps_enabled() defense clause, add a hook wp_doing_sitemap.

For example

if ( ! $this->sitemaps_enabled() ) {
        $wp_query->set_404();
        status_header( 404 );
        return;
}

// Render stylesheet if this is stylesheet route.
if ( $stylesheet_type ) {
        ...

Becomes

if ( ! $this->sitemaps_enabled() ) {
        $wp_query->set_404();
        status_header( 404 );
        return;
}

do_action( 'wp_doing_sitemaps' );

// Render stylesheet if this is stylesheet route.
if ( $stylesheet_type ) {
        ...

Add a wp_doing_sitemaps action hook to WP_Sitemaps::render_sitemaps() to allow plugins to modify HTTP headers or set up state before sitemap output begins.

Trac ticket: https://core.trac.wordpress.org/ticket/60552

## Use of AI Tools
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[masteradhoc]

Hey @Cybr
Can you check the proposed PR (basically your suggestion) and see if this still suits your needs?

LGTM.

We may also want to consider adding a WP_Query variable to improve cohesion. See: https://core.trac.wordpress.org/ticket/51117#comment:7.

[westonruter]

Please provide an example of what this enabled which can't be done with current hooks combined with the new is_sitemap() function.

[westonruter]

If you want to send your own header in sitemap responses, you can do so as with the following, due to #51543:

<?php
add_action(
        'send_headers',
        function () {
                if ( function_exists( 'is_sitemap' ) && is_sitemap() ) {
                        header( 'X-My-Sitemap-Header: true' );
                }
        }
);

[adrianduffell]

This was discussed in today's bug scub. It may still be possible to make it in the 7.1 beta cycle but needs a final review.

[westonruter]

I'm closing as maybelater because I haven't seen that the desired result can't be achieved with existing hooks, per my comment.

As this has been closed as maybelater i'll close this PR for the moment. Happy to reopen anytime.