Custom fields on invoice line items

Add support for custom attributes on invoice line items (e.g. size, colour, SKU, material, dimensions) so each item can carry information beyond the default name, quantity, and rate fields. Useful for users selling physical goods with variants, or anyone whose work needs structured detail per line item that the standard fields don't capture. Different from reusable products and services (which is about saving items to a catalog for reuse). The two could work together, where custom attributes are defined once on a saved product and pulled in automatically.