Function next_data 

pub async fn next_data<F, Fut, D>(
    new_data: NewData,
    state: StreamState,
    page_size: usize,
    get_data: F,
) -> Option<(Vec<Result<D>>, StreamState)>
where
    F: Fn(GetData) -> Fut,
    Fut: Future<Output = Result<Vec<D>>>,

Get the next data in the stream.