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/frame/election-provider-multi-phase/src/mock.rs

@@ -27,7 +27,7 @@ use parking_lot::RwLock;
 use sp_core::{
 	offchain::{
 		testing::{PoolState, TestOffchainExt, TestTransactionPoolExt},
-		OffchainExt, TransactionPoolExt,
+		OffchainDbExt, OffchainWorkerExt, TransactionPoolExt,
 	},
 	H256,
 };
@@ -369,7 +369,8 @@ impl ExtBuilder {
 		seed[0..4].copy_from_slice(&iters.to_le_bytes());
 		offchain_state.write().seed = seed;
 
-		ext.register_extension(OffchainExt::new(offchain));
+		ext.register_extension(OffchainDbExt::new(offchain.clone()));
+		ext.register_extension(OffchainWorkerExt::new(offchain));
 		ext.register_extension(TransactionPoolExt::new(pool));
 
 		(ext, pool_state)