Introduce ChainSyncInterface (#12489)

* Introduce `ChainSyncInterface`

`ChainSyncInterface` provides an asynchronous interface for other
subsystems to submit calls to `ChainSync`. This allows `NetworkService`
to delegate calls to `ChainSync` while still providing the same API
for other subsystems (for now). This makes it possible to move the
syncing code in piecemeal fashion out of `protocol.rs` as the calls
are just forwarded to `ChainSync`.

* Apply review comments

* Fix tests

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

@@ -864,7 +864,7 @@ where
 		let block_announce_validator = config
 			.block_announce_validator
 			.unwrap_or_else(|| Box::new(DefaultBlockAnnounceValidator));
-		let chain_sync = ChainSync::new(
+		let (chain_sync, chain_sync_service) = ChainSync::new(
 			match network_config.sync_mode {
 				SyncMode::Full => sc_network_common::sync::SyncMode::Full,
 				SyncMode::Fast { skip_proofs, storage_chain_mode } =>
@@ -902,6 +902,7 @@ where
 			fork_id,
 			import_queue,
 			chain_sync: Box::new(chain_sync),
+			chain_sync_service,
 			metrics_registry: None,
 			block_announce_config,
 			block_request_protocol_config,