Add Support for Endpoint Prefix (#420)

* Add Support for Endpoint Prefix

I decided to allow the httpLabel fields to remain optional for now (in the future, hopefully we'll be able to code generate on their required status and delete some code!).

In other exciting news, there are now no failing protocol tests!

* Simplify code and add test for unset

* Update codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/generators/EndpointTraitBindingGenerator.kt

* Fix clippy errors