examples: build system: add link-time registration plugin example

Closes https://github.com/espressif/esp-idf/issues/7682

examples/build_system/cmake/plugins/main/plugins_example_main.c

@@ -0,0 +1,13 @@
+/*
+ * SPDX-FileCopyrightText: 2022 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+#include <stdio.h>
+#include "plugins_api.h"
+
+void app_main(void)
+{
+    example_plugins_list();
+    example_plugins_greet("World");
+}