invoice + some table name thing

common/proto/table_definition.proto

@@ -216,6 +216,11 @@ message InvoiceTemplateFieldBinding {
   string collection_path = 2;
   string column_name = 3;
   InvoiceTemplateAccountingRole accounting_role = 4;
+
+  // Optional source reached through column_name when that column is a LINK.
+  // Both names are empty for a value stored directly on the invoice table.
+  string source_table_name = 5;
+  string source_column_name = 6;
 }
 
 // One physical dynamic table created for an invoice template scope. The root