Custom RPC implementation for node. (#3109)

* Allow RPCs to be customized.

* Implement node-rpc extensions.

* Working on a test.

* Add node-testing crate.

* Fix genesis test config

* Fix nonce lookups.

* Clean up.

* Fix expected block type.

* Make the RPC extension function optional.

* Fix service doc test.

* Bump jsonrpc.

* Bump client version.

* Update Cargo.lock

* Update jsonrpc.

* Fix build.

* Remove unused imports.

* Fix signed extra.

* Post merge clean up.

* Fix tests.

* Patch hashmap-core.

* Fix build.

* Fix build.

* Remove hashmap_core patches.

substrate/node-template/src/service.rs

@@ -263,5 +263,6 @@ construct_service_factory! {
 		FinalityProofProvider = { |client: Arc<FullClient<Self>>| {
 			Ok(Some(Arc::new(GrandpaFinalityProofProvider::new(client.clone(), client)) as _))
 		}},
+		RpcExtensions = (),
 	}
 }