Upload: Introduce `pre_wp_unique_filename_file_list` filter to allow for...

Upload: Introduce `pre_wp_unique_filename_file_list` filter to allow for short-circuiting the `scandir()` call in `wp_unique_filename()`.

This allows plugins to override the file fetching behavior to provide performance improvements for large directories.

Props joehoyle.
Fixes #50587.
Built from https://develop.svn.wordpress.org/trunk@48369


git-svn-id: https://core.svn.wordpress.org/trunk@48138 1a063a9b-81f0-0310-95a4-ce76da25c4cd