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

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

examples/build_system/cmake/plugins/components/plugins/include/plugins_api.h

@@ -0,0 +1,48 @@
+/*
+ * SPDX-FileCopyrightText: 2022 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* This stucture describes the plugin to the rest of the application */
+typedef struct {
+    /* A pointer to the plugin name */
+    const char* name;
+
+    /* A function which the plugin provides to the application.
+     * In this example, this function prints something to the console
+     * depending on the value of the argument 'arg'.
+     */
+    void (*greet)(const char* arg);
+} example_plugin_desc_t;
+
+/**
+ * @brief Register the plugin with the application
+ * This function is called from each plugin's "constructor" function.
+ * It adds the plugin to the list.
+ * @param plugin_desc  Pointer to the structure which describes the given plugin.
+ */
+void example_plugin_register(const example_plugin_desc_t* plugin_desc);
+
+/**
+ * @brief Print the list of registered plugins to the console.
+ * This function is called from the application.
+ */
+void example_plugins_list(void);
+
+/**
+ * @brief Invoke 'greet' function of each registered plugin with the given argument.
+ * This function is called from the application.
+ * @param arg  argument to pass to plugins' greet functions.
+ */
+void example_plugins_greet(const char* arg);
+
+
+#ifdef __cplusplus
+}
+#endif