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/src/protocol.rs

@@ -947,18 +947,6 @@ where
 		self.chain_sync.clear_justification_requests();
 	}
 
-	/// Request syncing for the given block from given set of peers.
-	/// Uses `protocol` to queue a new block download request and tries to dispatch all pending
-	/// requests.
-	pub fn set_sync_fork_request(
-		&mut self,
-		peers: Vec<PeerId>,
-		hash: &B::Hash,
-		number: NumberFor<B>,
-	) {
-		self.chain_sync.set_sync_fork_request(peers, hash, number)
-	}
-
 	/// A batch of blocks have been processed, with or without errors.
 	/// Call this when a batch of blocks have been processed by the importqueue, with or without
 	/// errors.
@@ -1461,8 +1449,11 @@ where
 			self.pending_messages.push_back(event);
 		}
 
-		// Check if there is any block announcement validation finished.
-		while let Poll::Ready(result) = self.chain_sync.poll_block_announce_validation(cx) {
+		// Advance the state of `ChainSync`
+		//
+		// Process any received requests received from `NetworkService` and
+		// check if there is any block announcement validation finished.
+		while let Poll::Ready(result) = self.chain_sync.poll(cx) {
 			match self.process_block_announce_validation_result(result) {
 				CustomMessageOutcome::None => {},
 				outcome => self.pending_messages.push_back(outcome),