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/lib.rs

@@ -279,6 +279,7 @@ pub use service::{
 	DecodingError, Keypair, NetworkService, NetworkWorker, NotificationSender,
 	NotificationSenderReady, OutboundFailure, PublicKey,
 };
+use sp_runtime::traits::{Block as BlockT, NumberFor};
 
 pub use sc_peerset::ReputationChange;
 
@@ -293,3 +294,14 @@ const MAX_CONNECTIONS_PER_PEER: usize = 2;
 
 /// The maximum number of concurrent established connections that were incoming.
 const MAX_CONNECTIONS_ESTABLISHED_INCOMING: u32 = 10_000;
+
+/// Abstraction over syncing-related services
+pub trait ChainSyncInterface<B: BlockT>:
+	NetworkSyncForkRequest<B::Hash, NumberFor<B>> + Send + Sync
+{
+}
+
+impl<T, B: BlockT> ChainSyncInterface<B> for T where
+	T: NetworkSyncForkRequest<B::Hash, NumberFor<B>> + Send + Sync
+{
+}