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/sync/src/service/mock.rs

@@ -0,0 +1,31 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2022 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+use libp2p::PeerId;
+use sc_network_common::service::NetworkSyncForkRequest;
+use sp_runtime::traits::{Block as BlockT, NumberFor};
+
+mockall::mock! {
+	pub ChainSyncInterface<B: BlockT> {}
+
+	impl<B: BlockT + 'static> NetworkSyncForkRequest<B::Hash, NumberFor<B>>
+		for ChainSyncInterface<B>
+	{
+		fn set_sync_fork_request(&self, peers: Vec<PeerId>, hash: B::Hash, number: NumberFor<B>);
+	}
+}