Companion for: pallet-mmr: move offchain logic to client-side gadget (#6321)

* Spawn MMR gadget when offchain indexing is enabled

* companion PR code review changes: 1st iteration

* Code review changes: 2nd iteration

* update lockfile for {"substrate"}

Co-authored-by: acatangiu <adrian@parity.io>
Co-authored-by: parity-processbot <>

polkadot/runtime/test-runtime/src/lib.rs

@@ -930,6 +930,10 @@ sp_api::impl_runtime_apis! {
 			Err(mmr::Error::PalletNotIncluded)
 		}
 
+		fn mmr_leaf_count() -> Result<mmr::LeafIndex, mmr::Error> {
+			Err(mmr::Error::PalletNotIncluded)
+		}
+
 		fn generate_proof(
 			_block_numbers: Vec<BlockNumber>,
 			_best_known_block_number: Option<BlockNumber>,