message: Add boxed method to service builder
diff: @@ -279,7 +279,7 @@ class ServerServiceGeneratorV2(
  rustTemplate(
  """
  ##[derive(Clone)]
  - pub struct $serviceName<S> {
  + pub struct $serviceName<S = #{SmithyHttpServer}::routing::Route> {
  router: #{SmithyHttpServer}::routers::RoutingService<#{Router}<S>, #{Protocol}>,
  }
  @@ -321,6 +321,21 @@ class ServerServiceGeneratorV2(
  router: self.router.map(|s| s.layer(layer))
  }
  }
  +
  + /// Applies [`Route::new`](#{SmithyHttpServer}::routing::Route::new) to all routes.
  + ///
  + /// This has the effect of erasing all types accumulated via [`layer`].
  + pub fn boxed<B>(self) -> $serviceName<#{SmithyHttpServer}::routing::Route<B>>
  + where
  + S: #{Tower}::Service<
  + #{Http}::Request<B>,
  + Response = #{Http}::Response<#{SmithyHttpServer}::body::BoxBody>,
  + Error = std::convert::Infallible>,
  + S: Clone + Send + 'static,
  + S::Future: Send + 'static,
  + {
  + self.layer(&#{Tower}::layer::layer_fn(#{SmithyHttpServer}::routing::Route::new))
  + }
  }
  impl<B, RespB, S> #{Tower}::Service<#{Http}::Request<B>> for $serviceName<S>
  