Adds a sender address and emails a confirmation code to it. The sender is returned with 201 as pending;
pass the code to POST /v2/senders/{id}/confirm before sending from it. A new sender needs the code even
when its domain is a verified sending domain. When the account already has the address, it is returned with
200 instead and nothing is emailed, so the call is safe to retry; use
POST /v2/senders/{id}/confirmation_code to send the code again.