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/primitives/core/src/lib.rs

@@ -119,9 +119,10 @@ impl ExecutionContext {
 		match self {
 			Importing | Syncing | BlockConstruction =>
 				offchain::Capabilities::none(),
-			// Enable keystore and transaction pool by default for offchain calls.
+			// Enable keystore, transaction pool and Offchain DB reads by default for offchain calls.
 			OffchainCall(None) => [
 				offchain::Capability::Keystore,
+				offchain::Capability::OffchainDbRead,
 				offchain::Capability::TransactionPool,
 			][..].into(),
 			OffchainCall(Some((_, capabilities))) => *capabilities,