Add IPC support (#6348)

This is useful for both security and performance reasons. IPC is faster
than TCP, and it is subject to OS access controls.

substrate/client/service/src/config.rs

@@ -67,6 +67,8 @@ pub struct Configuration {
 	pub rpc_http: Option<SocketAddr>,
 	/// RPC over Websockets binding address. `None` if disabled.
 	pub rpc_ws: Option<SocketAddr>,
+	/// RPC over IPC binding path. `None` if disabled.
+	pub rpc_ipc: Option<String>,
 	/// Maximum number of connections for WebSockets RPC server. `None` if default.
 	pub rpc_ws_max_connections: Option<usize>,
 	/// CORS settings for HTTP & WS servers. `None` if all origins are allowed.