List instantly available iMessage numbers

Phone numbers the provider has already registered and can assign on the spot. Order one by passing its id as availableNumberId to POST /v1/imessage/senders/order: the sender activates immediately instead of after the usual provisioning wait. Reserve it first with POST /v1/imessage/senders/available-numbers/{numberId}/reserve while the buyer decides. The list is a snapshot; a number can be taken between listing and ordering.