ruff_db: add "secondary" messages to Diagnostic trait

This is a small little hack to make the `Diagnostic` trait
capable of supporting attaching multiple spans.

This design should be considered transient. This was just the
quickest way that I could see to pass multiple spans through from
the type checker to the diagnostic renderer.

crates/red_knot_python_semantic/src/suppression.rs

@@ -325,6 +325,7 @@ impl<'a> CheckSuppressionsContext<'a> {
             range,
             severity,
             file: self.file,
+            secondary_messages: vec![],
         });
     }
 }