add 'pre_wp_get_attachment_url' filter

[fodisery]

We are making plugin for featured image using external url.

If user gt featured image url via wp_get_attachment_url() there is no way for us to hook because wp_get_attachment_url hook will never called if the post is not 'attachment'. The only way for us to make it work via creating fake attachment which is complicated.

pre_wp_get_attachment_url filter will be very helpful in this case.

[fodisery]

Real life problem:

​https://wordpress.org/plugins/featured-image-from-url/
They solve this by creating fake attachment for each featured image, which waste of space.

​https://wordpress.org/plugins/featured-image-by-url/
They stated plugin wont work if theme uses 'wp_get_attachment_url' to get featured image url

[fodisery]

even jetpack also have problem hooking photon to wp_get_attachment_url()
​https://github.com/Automattic/jetpack/issues/946

Adds pre_wp_get_attachment_url filter to short-circuit the wp_get_attachment_url function (allows better use of CDN, shared media library, external URLs, etc).

Trac ticket: https://core.trac.wordpress.org/ticket/57760