Sync: Propagate block announcement data (#7903)

* Sync: Propagate block announcement data

This pr adds a feature to the sync protocol to propagate the data that
we received alongside a block announcement. This is done by adding a
cache that caches the last X block announcement data where X is set to
the number of `in_peers` (giving every peer the chance to send us a
different block). This will be required by parachains to ensure that
even peers who are not connected to a collator receive the data
alongside the block announcement to properly validate it and request the
block.

* Review comment

* Bring back the code and add new variant to ensure we don't insert block
announce data when something wasn't checked

* Also use out_peers

substrate/client/finality-grandpa/src/communication/mod.rs

@@ -722,7 +722,7 @@ impl<Block: BlockT> Sink<Message<Block>> for OutgoingMessages<Block>
 			);
 
 			// announce the block we voted on to our peers.
-			self.network.lock().announce(target_hash, Vec::new());
+			self.network.lock().announce(target_hash, None);
 
 			// propagate the message to peers
 			let topic = round_topic::<Block>(self.round, self.set_id);

substrate/client/finality-grandpa/src/communication/tests.rs

@@ -68,7 +68,7 @@ impl sc_network_gossip::Network<Block> for TestNetwork {
 		let _ = self.sender.unbounded_send(Event::WriteNotification(who, message));
 	}
 
-	fn announce(&self, block: Hash, _associated_data: Vec<u8>) {
+	fn announce(&self, block: Hash, _associated_data: Option<Vec<u8>>) {
 		let _ = self.sender.unbounded_send(Event::Announce(block));
 	}
 }