Pass an executor through the Configuration (#4688)

* Pass an executor through the Configuration

* Make tasks_executor mandatory

* Fix tests

substrate/utils/browser/src/lib.rs

@@ -52,6 +52,9 @@ where
 		allow_private_ipv4: true,
 		enable_mdns: false,
 	};
+	config.tasks_executor = Some(Box::new(move |fut| {
+		wasm_bindgen_futures::spawn_local(fut)
+	}));
 	config.telemetry_external_transport = Some(transport);
 	config.roles = Roles::LIGHT;
 	config.name = format!("{} (Browser)", name);