Add Inventory Transfer

Add support for `InventoryTransfer` ([docs](https://system.netsuite.com/help/helpcenter/en_US/Output/Help/SuiteCloudCustomizationScriptingWebServices/SuiteTalkWebServices/InventoryTransfer.html), [schema](https://system.netsuite.com/help/helpcenter/en_US/srbrowser/Browser2014_1/schema/record/inventorytransfer.html)) and supporting classes.