Get the correct number of connected peers in SyncState (#13700)

`Protocol` is not a reliable source for the information of connected
peers because it doesn't have real-time information of the actual
connectivity state because it's not resposible for accepting/rejecting
connections and gets that information with delay from `SyncinEngine`.

substrate/client/network/common/src/sync.rs

@@ -94,6 +94,8 @@ pub struct SyncStatus<Block: BlockT> {
 	pub best_seen_block: Option<NumberFor<Block>>,
 	/// Number of peers participating in syncing.
 	pub num_peers: u32,
+	/// Number of peers known to `SyncingEngine` (both full and light).
+	pub num_connected_peers: u32,
 	/// Number of blocks queued for import
 	pub queued_blocks: u32,
 	/// State sync status in progress, if any.