Regression in 0.57 where default generic for service struct was removed

[david-perez]

Prior to 0.57, one could build and return a service from a function like this:

pub fn build_service() -> PokemonService {
    let service = PokemonService::builder_without_plugins()
        .operation(operation_handler)
        .build()
        .expect("unable to build service");

    service.boxed()
}

This is because we had a default generic on the service struct. But since #3095, this no longer compiles, because we accidentally removed that default generic:

So even in the most common use case where the user doesn't want to be generic over the body type and was satisfied with aws_smithy_http_server::body::Body, they now need to write this function as:

pub fn build_service() -> PokemonService<
    aws_smithy_http_server::routing::RoutingService<
      aws_smithy_http_server::protocol::rest::router::RestRouter<
        aws_smithy_http_server::routing::Route<aws_smithy_http_server::body::Body>,
      >,
      aws_smithy_http_server::protocol::rest_json_1::RestJson1,
    >,
> {
    let service = PokemonService::builder_without_plugins()
        .operation(operation_handler)
        .build()
        .expect("unable to build service");

    service.boxed()
}

This is a regression we didn't catch because we don't have any tests in our suite that initialize the service within a function. We should keep providing a default for the S generic for the most common use case.