Backport: Implement basic equivocations detection loop (#2375)

* Implement basic equivocations detection loop (#2367)

* FinalityProofsBuf adjustments

- store a Vec<FinalityProof>
- transform prune `buf_limit` to Option

* FinalityProof: add target_header_hash()

* Target client: implement best_synced_header_hash()

* Implement first version of the equivocations detection loop

* Address code review comments

* Leftover

* polkadot-staging adjustments

bridges/relays/finality/src/lib.rs

@@ -22,6 +22,7 @@
 pub use crate::{
 	base::{FinalityPipeline, SourceClientBase},
 	finality_loop::{metrics_prefix, run, FinalitySyncParams, SourceClient, TargetClient},
+	finality_proofs::{FinalityProofsBuf, FinalityProofsStream},
 	sync_loop_metrics::SyncLoopMetrics,
 };