Custom RPC for Merkle Mountain Range pallet (#8137)

* Add MMR custom RPC.

* Change RuntimeApi to avoid hardcoding leaf type.

* Properly implement the new RuntimeAPI and wire up RPC.

* Extract Offchain DB as separate execution extension.

* Enable offchain DB access for offchain calls.

* Fix offchain_election tests.

* Skip block initialisation for proof generation.

* Fix integration test setup.

* Fix offchain tests. Not sure how I missed them earlier 🤷.

* Fix long line.

* One more test missing.

* Update mock for multi-phase.

* Address review grumbbles.

* Address review grumbles.

* Fix line width of a comment

substrate/bin/node/testing/src/bench.rs

@@ -417,13 +417,14 @@ impl BenchDb {
 		};
 		let task_executor = TaskExecutor::new();
 
-		let (client, backend) = sc_service::new_client(
-			db_config,
+		let backend = sc_service::new_db_backend(db_config).expect("Should not fail");
+		let client = sc_service::new_client(
+			backend.clone(),
 			NativeExecutor::new(WasmExecutionMethod::Compiled, None, 8),
 			&keyring.generate_genesis(),
 			None,
 			None,
-			ExecutionExtensions::new(profile.into_execution_strategies(), None),
+			ExecutionExtensions::new(profile.into_execution_strategies(), None, None),
 			Box::new(task_executor.clone()),
 			None,
 			Default::default(),