Function build_default_block_downloader 

pub fn build_default_block_downloader<Block, Client, Net>(
    protocol_id: &ProtocolId,
    fork_id: Option<&str>,
    net_config: &mut FullNetworkConfiguration<Block, <Block as BlockT>::Hash, Net>,
    network_service_handle: NetworkServiceHandle,
    client: Arc<Client>,
    num_peers_hint: usize,
    spawn_handle: &SpawnTaskHandle,
) -> Arc<dyn BlockDownloader<Block>>
where
    Block: BlockT,
    Client: HeaderBackend<Block> + BlockBackend<Block> + Send + Sync + 'static,
    Net: NetworkBackend<Block, <Block as BlockT>::Hash>,

Build default block downloader