Privacy: Introduce `wp_privacy_additional_user_data` filter to make it easier...

Privacy: Introduce `wp_privacy_additional_user_data` filter to make it easier to include additional user meta in a personal data export.

Props pbiron, xkon, garrett-eclipse, azaozz.
Fixes #47509.
Built from https://develop.svn.wordpress.org/trunk@47270


git-svn-id: https://core.svn.wordpress.org/trunk@47070 1a063a9b-81f0-0310-95a4-ce76da25c4cd