expose jsonrpc-core client (#672)

* expose jsonrpc-core client

* use shared reference to RpcClienT

* don't expose Arc<dyn RpcClientT>

* cargo fmt

examples/examples/custom_rpc_client.rs

@@ -88,7 +88,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     // Pass this into our OnlineClient to instantiate it. This will lead to some
     // RPC calls being made to fetch chain details/metadata, which will immediately
     // fail..
-    let _ = OnlineClient::<PolkadotConfig>::from_rpc_client(rpc_client).await;
+    let _ = OnlineClient::<PolkadotConfig>::from_rpc_client(Arc::new(rpc_client)).await;
 
     // But, we can see that the calls were made via our custom RPC client:
     println!("Log of calls made:\n\n{}", log.lock().unwrap().as_str());