Prime users cache in WP_Query and post REST API controller.

For a call to `WP_Query` or a post REST API request that contains posts from multiple authors, call the `cache_users` function, to ensure that all user data for post authors is primed in 
a single database query. This results in far fewer database queries on multiple author sites. 

Props spacedmonkey, timothyblynjacobs, peterwilsoncc.
Fixes #55716.


git-svn-id: https://develop.svn.wordpress.org/trunk@53482 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -7474,6 +7474,20 @@ function update_post_caches( &$posts, $post_type = 'post', $update_term_cache =
 	}
 }
 
+/**
+ * Prime post author user caches.
+ *
+ * @since 6.1.0
+ *
+ * @param WP_Post[] $posts Array of Post objects
+ */
+function update_post_author_caches( $posts ) {
+	$author_ids = wp_list_pluck( $posts, 'post_author' );
+	$author_ids = array_map( 'absint', $author_ids );
+	$author_ids = array_unique( array_filter( $author_ids ) );
+	cache_users( $author_ids );
+}
+
 /**
  * Updates metadata cache for list of post IDs.
  *