fix(sdk): fix builder pattern to actually work with default values

In previous versions the builder pattern required all values to be set.
This has not been fixed, so that default values are allowed.

crates/dagger-codegen/src/rust/templates/object_tmpl.rs

@@ -86,7 +86,7 @@ pub fn render_optional_field_args(
         }
         quote! {
             $(a.description.pipe(|d| format_struct_comment(d)))
-            #[builder(setter(into, strip_option))]
+            #[builder(setter(into, strip_option), default)]
             pub $(format_struct_name(&a.name)): Option<$(type_)>,
         }
     });