Add BEEFY latestFinalized RPC and deduplicate code between BEEFY and GRANDPA (#10568)

* beefy: add dummy latest_finalized() RPC

* beefy: rpc latest_best_beefy() using shared mem

* beefy: rpc populate latest_best_beefy()

* beefy: rpc handle readiness

* beefy: best block over channel - wip

Not working because channel can't be simply opened and receiver passed
to `rpc_extensions_builder` because `rpc_extensions_builder` has to be
`Fn` and not `FnOnce`... and and Receiver side of mpsc can't be cloned

yay!..

* beefy: make notification channels payload-agnostic

* beefy: use notification mechanism instead of custom channel

* beefy: add tracing key to notif channels

* sc-utils: add notification channel - wip

* beefy: use sc-utils generic notification channel

* grandpa: use sc-utils generic notification channel

* fix grumbles

* beefy-rpc: get best block header instead of number

* beefy-rpc: rename to `beefy_getFinalizedHead`

* fix nitpicks

* client-rpc-notifications: move generic Error from struct to fn

* beefy: use header from notification instead of getting from database

* beefy-rpc: get best block hash instead of header

* beefy-rpc: fix and improve latestHead test

* beefy-rpc: bubble up errors from rpc-handler instantiation

* update lockfile

* Apply suggestions from code review

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* fix errors and warnings

* fix nit

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

substrate/client/beefy/src/lib.rs

@@ -31,6 +31,8 @@ use sp_runtime::traits::Block;
 
 use beefy_primitives::BeefyApi;
 
+use crate::notification::{BeefyBestBlockSender, BeefySignedCommitmentSender};
+
 mod error;
 mod gossip;
 mod keystore;
@@ -121,7 +123,9 @@ where
 	/// Gossip network
 	pub network: N,
 	/// BEEFY signed commitment sender
-	pub signed_commitment_sender: notification::BeefySignedCommitmentSender<B>,
+	pub signed_commitment_sender: BeefySignedCommitmentSender<B>,
+	/// BEEFY best block sender
+	pub beefy_best_block_sender: BeefyBestBlockSender<B>,
 	/// Minimal delta between blocks, BEEFY should vote for
 	pub min_block_delta: u32,
 	/// Prometheus metric registry
@@ -147,6 +151,7 @@ where
 		key_store,
 		network,
 		signed_commitment_sender,
+		beefy_best_block_sender,
 		min_block_delta,
 		prometheus_registry,
 		protocol_name,
@@ -174,6 +179,7 @@ where
 		backend,
 		key_store: key_store.into(),
 		signed_commitment_sender,
+		beefy_best_block_sender,
 		gossip_engine,
 		gossip_validator,
 		min_block_delta,