pub fn executor<E>(mut self, executor: E) -> Self
    where
        E: Executor<Pin<Box<dyn Future<Output = ()> + Send>>> + Send + Sync + 'static,
    {
        self.executor = SharedExec::new(executor);
        self
    }
Sets the executor used to spawn async tasks.

Uses `tokio::spawn` by default.
tonic/src/transport/channel/endpoint.rs