Fix ProductGetters type to not assume API return structure

[filipsobol]

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Currently, we assume that the products passed to getAttributes and getFiltered methods in ProductGetters is an array of PRODUCT, but the response from the API could be of any type. We should add a new generic to this interface that will define the structure of response.

https://github.com/vuestorefront/vue-storefront/blob/next/packages/core/core/src/types.ts#L503-L519