Add new service builder codegen (#1693)

* Add `ServerProtocol` interface to allow for server side protocol specific methods.

* Make public the structs merged in https://github.com/awslabs/smithy-rs/pull/1679.

* Add `ServerOperationGenerator`, which generates a ZST and implements `OperationShape` on it.

* Add `ServerServiceGeneratorV2`, which generates the service newtype around a router and a service builder.

* Add `hidden` argument to `RustModule` which allows modules to be marked with `#[doc(hidden)]`.

* Add `BuildModifier` trait to provide a common interface for extending service builders.

* Add `Upgradable` trait to simplifying bounds when upgrading from an `Operation` to a HTTP service.

* Add `FromRequest`, `FromParts`, and `IntoResponse` implementations.

* Make `RoutingService` accept general body types `B` for the inner services `http::Response<B>`.

* Use new service builder in protocol tests.