Attach an additional email address to a Greenhouse user — the V3 replacement for V1 POST /users/{id}/email_addresses. This only adds secondary addresses; it cannot change the user's primary email or replace an existing address. Emails are globally unique across users, so attempting to attach an address that already belongs to anyone returns 409. Re-posting an address that already exists on the target user with send_verification=true while it is still unverified resends the verification email and returns 200 instead of 201.