Clippy warnings

Clippy warning on &String parameters

If your .smithy model has an operation whose input parameter is a 'String', clippy may generate the following warning:

warning: writing `&String` instead of `&str` involves a new object where a slice will do

That's incorrect for our use case (try changing it and you'll see that change &String to &str doesn't work). To avoid this warning, add #![allow(clippy::ptr_arg)] as the first line in your interface project's src/lib.rs