Function client_streaming_handler_fn 

pub fn client_streaming_handler_fn<F, Fut, Req, Res, B>(
    f: F,
) -> FnClientStreamingHandler<F>
where
    F: Fn(RequestContext, ServiceStream<Req>) -> Fut + Send + Sync + 'static,
    Fut: Future<Output = ServiceResult<B>> + Send + 'static,
    Req: Message + Send + 'static,
    Res: Message + Send + 'static,
    B: Encodable<Res> + Send + 'static,

Helper function to create a client streaming handler from an async function.