bubble up download event from engineHandler

[mattsse]

we're currently ignoring events for new downloads

reth/crates/engine/tree/src/engine.rs

Lines 105 to 110 in c3b334b

	// advance the downloader
	if let Poll::Ready(DownloadOutcome::Blocks(blocks)) = self.downloader.poll(cx) {
	// delegate the downloaded blocks to the handler
	self.handler.on_event(FromEngine::DownloadedBlocks(blocks));
	continue
	}

we can introduce a new event type that wraps:

reth/crates/engine/tree/src/engine.rs

Line 64 in c3b334b

type Event = T::Event;

and also has a variant for new download blocks event:

reth/crates/engine/tree/src/download.rs

Lines 42 to 43 in c3b334b

	/// New download started.
	NewDownloadStarted { remaining_blocks: u64, target: B256 },