Media: Store attachment’s file size in metadata.

Store the file size of all newly uploaded attachments, as part of the metadata stored in post meta. Storing file size means, developers will not have to resort to doing `filesize` function calls, that can be time consuming on assets on offloaded to services like Amazon’s S3. 

This change also introduces a new helper function called, `wp_filesize`. This is a wrapper around the `filesize` php function, that adds some helpful filters and ensures the return value is an integer.

Props Cybr, Spacedmonkey, SergeyBiryukov, johnwatkins0, swissspidy, desrosj, joemcgill, azaozz, antpb, adamsilverstein, uday17035. 
Fixes #49412. 



git-svn-id: https://develop.svn.wordpress.org/trunk@52837 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -6538,6 +6538,7 @@ function wp_delete_attachment_files( $post_id, $meta, $backup_sizes, $file ) {
  *     @type array  $sizes      Keys are size slugs, each value is an array containing
  *                              'file', 'width', 'height', and 'mime-type'.
  *     @type array  $image_meta Image metadata.
+ *     @type int    $filesize   File size of the attachment.
  * }
  */
 function wp_get_attachment_metadata( $attachment_id = 0, $unfiltered = false ) {