Add inventory_valuation_method to items endpooints

Add inventory_valuation_method to items endpooints

To ensure consistent item creation it would be helpful to have the inventory_valuation_method (FIFO vs WAC) be able to be set at item creation or as an update (consistent with current behavior where it is not allowed for items with existing transactions) via the API. 

Additionally, seeing the inventory_valuation_method when getting item data would also be helpful.