Changes to Send a mass message

the response property recipientCount became nullable for the status 201

added the new optional request property excludedLists/dynamicSmartListUuids

added the new optional request property includedLists/dynamicSmartListUuids

added the optional property status to the response with the 201 status

added the new optional header request parameter Idempotency-Key

added the non-success response with the status 409