Product page crashes with more than two variant attributes

[maarcingebala]

We shuold support rendering products with more than two variant attribtues. E.g this is a valid Saleor product with three variant attributes:

In the storefront it should be rendered as three separate dropdowns, since you need to specify each of those attributes to define a variant that you want to add to the cart.

The problem is that in the ProductDescription component we assume that there may be at most two attribute pickers:

https://github.com/mirumee/saleor-storefront/blob/a98138786e9c9560d8f76cd40a0a45f66ca21d7a/src/components/ProductDescription/index.tsx#L24-L25

The design for the new product page supporting more than two pickers: