Add equivocation detector crate and implement clients (#2348) (#2353)

* Split FinalitySyncPipeline and SourceClient

* Move some logic to finality_base

* Add empty equivocation detection clients

* Add equivocation reporting logic to the source client

* Use convenience trait for SubstrateFinalitySyncPipeline

* Define JustificationVerificationContext for GRANDPA

* Equivocation source client: finality_verification_context()

* Equivocation source client: synced_headers_finality_info()

* reuse HeaderFinalityInfo

* Define EquivocationsFinder

* Fix spellcheck

* Address review comments

* Avoid equivocations lookup errors

bridges/relays/finality/src/base.rs

@@ -0,0 +1,51 @@
+// Copyright 2019-2023 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common 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.
+
+// Parity Bridges Common 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 Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+use async_trait::async_trait;
+use bp_header_chain::FinalityProof;
+use futures::Stream;
+use relay_utils::relay_loop::Client as RelayClient;
+use std::fmt::Debug;
+
+/// Base finality pipeline.
+pub trait FinalityPipeline: 'static + Clone + Debug + Send + Sync {
+	/// Name of the finality proofs source.
+	const SOURCE_NAME: &'static str;
+	/// Name of the finality proofs target.
+	const TARGET_NAME: &'static str;
+
+	/// Synced headers are identified by this hash.
+	type Hash: Eq + Clone + Copy + Send + Sync + Debug;
+	/// Synced headers are identified by this number.
+	type Number: relay_utils::BlockNumberBase;
+	/// Finality proof type.
+	type FinalityProof: FinalityProof<Self::Number>;
+}
+
+/// Source client used in finality related loops.
+#[async_trait]
+pub trait SourceClientBase<P: FinalityPipeline>: RelayClient {
+	/// Stream of new finality proofs. The stream is allowed to miss proofs for some
+	/// headers, even if those headers are mandatory.
+	type FinalityProofsStream: Stream<Item = P::FinalityProof> + Send + Unpin;
+
+	/// Subscribe to new finality proofs.
+	async fn finality_proofs(&self) -> Result<Self::FinalityProofsStream, Self::Error>;
+}
+
+/// Target client used in finality related loops.
+#[async_trait]
+pub trait TargetClientBase<P: FinalityPipeline>: RelayClient {}