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/merkle-mountain-range/src/tests.rs

@@ -23,7 +23,7 @@ use sp_core::{
 	H256,
 	offchain::{
 		testing::TestOffchainExt,
-		OffchainExt,
+		OffchainWorkerExt, OffchainDbExt,
 	},
 };
 use pallet_mmr_primitives::{Proof, Compact};
@@ -34,7 +34,8 @@ pub(crate) fn new_test_ext() -> sp_io::TestExternalities {
 
 fn register_offchain_ext(ext: &mut sp_io::TestExternalities) {
 	let (offchain, _offchain_state) = TestOffchainExt::with_offchain_db(ext.offchain_db());
-	ext.register_extension(OffchainExt::new(offchain));
+	ext.register_extension(OffchainDbExt::new(offchain.clone()));
+	ext.register_extension(OffchainWorkerExt::new(offchain));
 }
 
 fn new_block() -> u64 {