Expose cross references in the nextgen inventory summary

[Kjetil Ringstad]

Can you please consider exposing product cross references in the nextgen inventory summary endpoint GET/api/v3/Inventory ?

If you could expose it the same way the attributes are exposed, a separate call to fetch these from the products are prevented. 

[Steinar Møller]

We also need crossReference for POST, in the current API and NextGen.