Check if updating usage of EnumTrait to EnumShape improves code generated by smithy-rs

[Velfi]

We currently have a lot of usages of if (shape.hasTrait<EnumTrait>()) { that may need to be converted to if (shape is EnumShape) {. Convert them and then verify that this has improved the generated code (i.e. if it generates less code or clearer code.)

[jdisanti]

It looks like Smithy's ModelTransformer has a changeStringEnumsToEnumShapes transform, which would allow us to migrate the codegen to only look at EnumShape and stop using EnumTrait entirely. We could just throw this into the CodegenVisitor's baselineTransform.

[david-perez]

There's also no behavioral change with regards to when invalid enum values are rejected in the server request deserialization path, so I think we should be good applying that model transform everywhere and not affect semantics, regardless of whether we're generating an IDL v1 or v2 model.

[jdisanti]

Confirmed with the Smithy folks that we should model transform from EnumTrait to EnumShape, and then refactor our EnumGenerator to work exclusively off the EnumShape.