Adds short circuit filter to wp_get_attachment_url

[liamfirth]

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

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[github-actions]

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[github-actions]

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[github-actions]

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