Support custom properties

[mattbrailsford]

Provide an API or config to allow custom properties to be assigned to an order / order line without needing to run a completely custom build. Maybe this could be a bit like the old TC HTML API where additional properties are defined in a hidden field and if that field is present, we process the field names defined and copy the values directly into the order.

Alternatively, we could maybe just fire a custom event and allow folks to hook in and post process the order, fetching the additional properties from the form and adding them to the order before the order is saved (Maybe this is even possible now? 🤔)

Something to think about in the new year