Privacy: Introduce filters for the headers of all the privacy-related e-mails:

* `wp_privacy_personal_data_email_headers`
* `user_request_confirmed_email_headers`
* `user_erasure_complete_email_headers`
* `user_request_action_email_headers`

Props xkon, garrett-eclipse, zaffarn, desrosj.
Fixes #44501.
Built from https://develop.svn.wordpress.org/trunk@47279


git-svn-id: https://core.svn.wordpress.org/trunk@47079 1a063a9b-81f0-0310-95a4-ce76da25c4cd