FIX: Make sc-network-sync types Public (paritytech#3586)

This PR makes `sc-network-sync` types public following
[this](paritytech#3556) issue.

Fixes paritytech#3556

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

@@ -28,7 +28,7 @@ mod justification_requests;
 mod pending_responses;
 mod request_metrics;
 mod schema;
-mod types;
+pub mod types;
 
 pub mod block_relay_protocol;
 pub mod block_request_handler;