Function build_openapi 

pub fn build_openapi<'openapi, F>(f: F) -> OpenApiWrapper<'openapi>
where
    F: Fn() -> OpenApiBuilder,

Build the openapi spec. This function should be called after all the endpoints are defined. Because the openapi spec is cached, it’s cheap to call this function multiple times. The f function is called only when the openapi spec is not built yet.