This commit adds `map` function for `ServiceConfig` to allow defining
normal services to `actix_web::web::ServiceConfig` via `ServiceConfig`
in similar fashion to `Scope` and `UtoipaApp`.

This example demonstrates the `map` functionality of `ServiceConfig` to
allow direct access to underlying `actix_web::web::ServiceConfig`.
```rust
fn config(cfg: &mut service_config::ServiceConfig) {
    cfg.service(handler3)
        .map(|config| config.service(normal_service));
}
```

Closes #1173