[Add] Allow generation of Ref for invoice object

[HaddadJoe]

This will allow generation of ref objects for the invoice class.

invoice = Invoice()
attachment = Attachable()  # object to create an attachment on quickbooks

attachable_ref = AttachableRef()  # object to attach references to the attachment object

attachable_ref.EntityRef = invoice.to_ref()
attachment.AttachableRef.append(attachable_ref)

attachment.FileName = "test.txt"
attachment.ContentType = "text/plain"
attachment._FilePath = f"/temp/dir/test.txt"  # full path to file
attachment.save(qb=quickbooks_client)