WHEN I request GET /entities/items/{item_id}
AND provide the _fields parameter containing allowed field names
THEN I get 200 HTTP response
AND response contains serialization of the item data following the specification, including ID and only the fields I have requested
AND headers of the response include the headers as defined in the specification

Task Breakdown Notes:

• in the RouteHandler, construct the GetItemRequest object based on both the ItemId and the _fields request parameter
• in the GetItem use case, filter the itemSerialization based on the _fields from the GetItemRequest
  • create a new GetItemFieldsFilter object that filters the itemSerialization based on requested fields
• since it is an array, the filtered ItemSerialization can still be passed into GetItemResult