Content Explorer: Add Custom Metadata Column

**Solution:**
Provide the ability to add custom metadata template property values as a column or to override the default columns in a json array property. For example; columnProps. This would allow the modification of some of the ItemList.Table.Column values found here:
https://github.com/box/box-ui-elements/blob/master/src/components/ContentExplorer/ItemList.js#L172

**Describe alternatives you've considered**
Manually modify the Table in the ItemList and add/remove columns. Realistically, its easier to create a custom list/table component rather than modify and maintain the UI Element.