Function register_api 

fn register_api<E, S, F, ModuleError, ModuleVersion>(
    path: &'static str,
    app: &mut App<S, E>,
    f: F,
) -> Result<()>
where
    S: 'static + Send + Sync,
    E: Send + Sync + 'static + Error + From<ModuleError>,
    ModuleError: Send + Sync + 'static,
    ModuleVersion: StaticVersionType + 'static,
    F: Fn(Version) -> Result<Api<S, ModuleError, ModuleVersion>>,

Registers two versions (v0 and v1) of the same API module under the given path.