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/service/src/lib.rs

@@ -248,7 +248,7 @@ async fn build_network_future<
 				};
 
 				if announce_imported_blocks {
-					network.service().announce_block(notification.hash, Vec::new());
+					network.service().announce_block(notification.hash, None);
 				}
 
 				if notification.is_new_best {