Add and remove item attribute conditions

Adds conditions for item attributes so that items can be filtered based on these conditions, from an Alternate category.
For example, a Primary category called Chairs has three chairs of different materials. An Alternate category called Wooden Chairs with Primary category Chairs as source will list all the three chairs. Through this endpoint you can add a 'item attribute condition' to filter chairs made of wood from Wooden Chairs. Another example: Using this endpoint, you can combine categories and list items on sale (where sale attribute is true).

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