Step 1 of FedEx MFA registration: pre-create the EasyPost
CarrierAccount and register the billing address.

EasyPost's MFA workflow requires easypost_details.carrier_account_id
on /fedex_registrations/{account}/address. The SDK's
carrier_account.create helper routes FedexAccount to the retired
/carrier_accounts/register endpoint, so we provision a stub via the
plain POST /carrier_accounts (see create_fedex_carrier_account_stub)
first, persist its id on the row, and thread it through the MFA
steps. EasyPost fills in credentials.account_number and
credentials.mfa_key as the user completes PIN validation.