Multisite: Add filter to allow custom form attributes in new site cre…

…ation.

This change introduces the `network_site_new_form_attributes` filter 
to the site creation form (`/wp-admin/network/site-new.php`) in WordPress Multisite.

The filter allows developers to modify form attributes such as `enctype="multipart/form-data"`, enabling file uploads or other custom behaviors when creating a new site.

This enhancement provides more flexibility for developers extending the multisite functionality without requiring modifications to core files.

Fixes #62086

src/wp-admin/network/site-new.php

@@ -204,7 +204,7 @@
 }
 ?>
 <p><?php echo wp_required_field_message(); ?></p>
-<form method="post" action="<?php echo esc_url( network_admin_url( 'site-new.php?action=add-site' ) ); ?>" novalidate="novalidate">
+<form method="post" action="<?php echo esc_url( network_admin_url( 'site-new.php?action=add-site' ) ); ?>" novalidate="novalidate"<?php echo apply_filters('network_new_site_form_attributes', ''); ?>>
 <?php wp_nonce_field( 'add-blog', '_wpnonce_add-blog' ); ?>
 	<table class="form-table" role="presentation">
 		<tr class="form-field form-required">