optional support

betterproto-extras/src/descriptor_pool.rs

@@ -68,6 +68,8 @@ fn create_cached_descriptor_in_pool<'py>(
 
             if meta.is_list_field(field_name)? {
                 field.set_label(Label::Repeated);
+            } else if field_meta.getattr("optional")?.extract::<bool>()? {
+                field.proto3_optional = Some(true);
             }
 
             if let Some(grp) = meta.oneof_group(field_name)? {