Add state object to cart modifier methods get_extra_cart_price_field and get_extra_cart_item_price_field

This pull request fixes an issue I had on my last shop project.
I needed a cart modifier which calculates a rebate according to a customer bonus state.
The customer identified himself by loggin into the shop, so the cart modifier needed a way to access the request object. The only viable solution I found, was to add the request object to the ``state``-object during the invocation of ``cart.update(state)``. However this ``state``-object never was passed down to the two cart modifiers methods ``get_extra_cart_price_field`` and ``get_extra_cart_item_price_field``.