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/core/rpc/src/metadata.rs

@@ -52,3 +52,9 @@ impl Metadata {
 		(rx, Self::new(tx))
 	}
 }
+
+impl From<mpsc::Sender<String>> for Metadata {
+	fn from(sender: mpsc::Sender<String>) -> Self {
+		Self::new(sender)
+	}
+}