I installed the gem for product assembly (https://github.com/solidusio-contrib/solidus_product_assembly), am running through setting up some bundles, but it looks like a bundle is tied to a single variant of a product. Is it possible to allow for the user to select a variant when purchasing the bundle? So if I wanted to have a mug, tshirt and poster bundle and wanted to let the customer pick their size of shirt or from multiple posters. Is that currently possible and I just missed how to do it? If not, is it on the road map?