feat(endpoints): Add endpoint select changed event.

app/include/zmk/events/endpoint_selection_changed.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2021 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr.h>
+
+#include <zmk/endpoints_types.h>
+#include <zmk/event_manager.h>
+
+struct zmk_endpoint_selection_changed {
+    enum zmk_endpoint endpoint;
+};
+
+ZMK_EVENT_DECLARE(zmk_endpoint_selection_changed);