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/test-utils/client/src/lib.rs

@@ -198,6 +198,7 @@ impl<Block: BlockT, Executor, Backend, G: GenesisInit> TestClientBuilder<Block,
 	) where
 		Executor: sc_client_api::CallExecutor<Block> + 'static,
 		Backend: sc_client_api::backend::Backend<Block>,
+		<Backend as sc_client_api::backend::Backend<Block>>::OffchainStorage: 'static,
 	{
 		let storage = {
 			let mut storage = self.genesis_init.genesis_storage();
@@ -225,6 +226,7 @@ impl<Block: BlockT, Executor, Backend, G: GenesisInit> TestClientBuilder<Block,
 			ExecutionExtensions::new(
 				self.execution_strategies,
 				self.keystore,
+				sc_offchain::OffchainDb::factory_from_backend(&*self.backend),
 			),
 			None,
 			ClientConfig {