Add attributes to more entities in VN

[erik-kle]

Allow the possibility to add attributes to more entities in VN, the same as inventory, customer etc. This would provide a greater way to extend and integrate with VN, and to build custom solutions in a more dynamic way.

The entities that would benefit from attributes are:

SalesOrder

SalesOrderLines

PurchaseOrder

PurchaseOrderLines

CustomerInvoice

[Elbert Dannenberg]

That is a geat wish for years. It gives a lot more flexability 

[omelhus]

Add Employee to this wish-list and add the option to filter as well.

[Trygve Storrønningen1]

This wish/idea also applies to /api/v3/SalesOrders