Migrate pallet-mmr to the new pallet attribute macro (#9181)

* Migrate pallet-mmr to the new pallet attribute macro

Signed-off-by: koushiro <koushiro.cqx@gmail.com>

* fix typo

Signed-off-by: koushiro <koushiro.cqx@gmail.com>

* use instance macro

Co-authored-by: thiolliere <gui.thiolliere@gmail.com>

substrate/frame/merkle-mountain-range/rpc/src/lib.rs

@@ -25,6 +25,7 @@ use codec::{Codec, Encode};
 use jsonrpc_core::{Error, ErrorCode, Result};
 use jsonrpc_derive::rpc;
 use serde::{Deserialize, Serialize};
+
 use sp_api::ProvideRuntimeApi;
 use sp_blockchain::HeaderBackend;
 use sp_core::Bytes;