Add a filter to `attachment_url_to_postid`

[mattwiebe]

attachment_url_to_postid doesn't work on WordPress.com, which breaks any Customizer upload controls from showing their current file. Rather than trying to make the function work to support our tricky uploads system, a filter should be introduced so we (and anyone else with an unconventional filesystem) can use fallbacks to find the attachment ID.

[azaozz]

In 31867:

Introduce attachment_url_to_postid filter to let plugins manage the uploads location better.
Props mattwiebe. Fixes #31717.