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

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

examples/build_system/cmake/plugins/components/plugin_hello/CMakeLists.txt

@@ -0,0 +1,3 @@
+idf_component_register(SRCS plugin_hello.c
+                       PRIV_REQUIRES plugins
+                       WHOLE_ARCHIVE)

examples/build_system/cmake/plugins/components/plugin_hello/plugin_hello.c

@@ -0,0 +1,45 @@
+/*
+ * SPDX-FileCopyrightText: 2022 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+#include <stdio.h>
+#include <string.h>
+#include "plugins_api.h"
+
+/**
+ * This is an example function implemented by the plugin.
+ */
+static void plugin_hello_greet(const char* arg)
+{
+    if (arg == NULL) {
+        return;
+    }
+    printf("Hello, %s!\n", arg);
+}
+
+/* The code below demonstates both static and dynamic registration approaches. */
+
+/**
+ * Static registration of this plugin can be achieved by defining the plugin description
+ * structure and placing it into .plugins_desc section.
+ * The name of the section and its placement is determined by linker.lf file in 'plugins' component.
+ */
+static const example_plugin_desc_t __attribute__((section(".plugins_desc"),used)) PLUGIN = {
+    .name = "Hello",
+    .greet = &plugin_hello_greet
+};
+
+/**
+ * Dynamic registration of this plugin can be achieved by calling plugin registration function
+ * ('example_plugin_register') from a "constructor" function. Constructor function is called automatically
+ * during application startup.
+ */
+static void __attribute__((constructor)) plugin_hello_self_register(void)
+{
+    printf("Hello plugin performing self-registration...\n");
+    example_plugin_register(&(example_plugin_desc_t){
+        .name = "Hello",
+        .greet = &plugin_hello_greet
+    });
+}