Allow custom thumbnail for components in the component library

Currently working on some column components for our design system but they have the option to start with no rows which leaves just the header cell. This cell is the same for some of the different types of columns making them appear the same in the component library even though the content is different once more cells are added.

It would be nice to customize the thumbnail shown to designers in the component library to give a better preview of the component when the default is not helpful.