Users: Add new hooks to filter retrieve password emails.
This change introduces two new hooks to help developers to filter retrieve password emails:
send_retrieve_password_email can be used to filter whether to send the retrieve password email;
retrieve_password_notification_email can be used to filter the contents of the reset password notification email sent to the user.
This changesets also adds unit tests for these new filters.
Props connapptivity, costdev, audrasjb, johnbillion.
Fixes #54690.