Add block announce validator. (#3346)

* Add `BlockAnnounceValidator` trait.

* Add associated data to block announcement.

* Make tests compile.

* Move validator into `sync.rs`.

* Smaller changes.

* Update core/network/src/protocol.rs

Co-Authored-By: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Update core/network/src/protocol.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Update core/network/src/test/sync.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Formatting.

* Remove assoc. data from `BlockImportNotification`.

* Use `Option<Vec<u8>>` for associated data.

* Update core/network/src/protocol/sync.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Fix type error.

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

@@ -127,7 +127,7 @@ pub trait Network<Block: BlockT>: Clone + Send + 'static {
 	fn report(&self, who: network::PeerId, cost_benefit: i32);
 
 	/// Inform peers that a block with given hash should be downloaded.
-	fn announce(&self, block: Block::Hash);
+	fn announce(&self, block: Block::Hash, associated_data: Vec<u8>);
 }
 
 /// Create a unique topic for a round and set-id combo.
@@ -197,8 +197,8 @@ impl<B, S, H> Network<B> for Arc<NetworkService<B, S, H>> where
 		self.report_peer(who, cost_benefit)
 	}
 
-	fn announce(&self, block: B::Hash) {
-		self.announce_block(block)
+	fn announce(&self, block: B::Hash, associated_data: Vec<u8>) {
+		self.announce_block(block, associated_data)
 	}
 }
 
@@ -727,7 +727,7 @@ impl<Block: BlockT, N: Network<Block>> Sink for OutgoingMessages<Block, N>
 			);
 
 			// send the target block hash to the background block announcer
-			self.announce_sender.send(target_hash);
+			self.announce_sender.send(target_hash, Vec::new());
 
 			// propagate the message to peers
 			let topic = round_topic::<Block>(self.round, self.set_id);