SavedStruct: Introduce saved_struct_get_metadata (#3392)

* SavedStruct: Replace saved_struct_get_payload_size with saved_struct_get_metadata
  This new function can obtain the magic value, version and payload size
  all at once. This makes the function useful e.g.
  for backwards compatibility.
* SavedStruct: const-qualify saved_struct_save
* Toolbox: add saved struct documentation

Co-authored-by: あく <alleteam@gmail.com>

lib/toolbox/saved_struct.h

@@ -1,3 +1,8 @@
+/**
+ * @file saved_struct.h
+ * @brief SavedStruct - data serialization/de-serialization
+ * 
+ */
 #pragma once
 
 #include <stdint.h>
@@ -8,14 +13,50 @@
 extern "C" {
 #endif
 
+/** Load data from the file in saved structure format
+ *
+ * @param[in]  path     The path to the file
+ * @param[out] data     Pointer to the memory where to load data
+ * @param[in]  size     The size of the data
+ * @param[in]  magic    The magic to embed into metadata
+ * @param[in]  version  The version to embed into metadata
+ *
+ * @return     true on success, false otherwise
+ */
 bool saved_struct_load(const char* path, void* data, size_t size, uint8_t magic, uint8_t version);
 
-bool saved_struct_save(const char* path, void* data, size_t size, uint8_t magic, uint8_t version);
-
-bool saved_struct_get_payload_size(
+/** Save data in saved structure format
+ *
+ * @param[in]  path     The path to the file
+ * @param[in]  data     Pointer to the memory where data
+ * @param[in]  size     The size of the data
+ * @param[in]  magic    The magic to embed into metadata
+ * @param[in]  version  The version to embed into metadata
+ *
+ * @return     true on success, false otherwise
+ */
+bool saved_struct_save(
     const char* path,
+    const void* data,
+    size_t size,
     uint8_t magic,
-    uint8_t version,
+    uint8_t version);
+
+/** Get SavedStructure file metadata
+ *
+ * @param[in]  path          The path to the file
+ * @param[out] magic         Pointer to store magic or NULL if you don't need it
+ * @param[out] version       Pointer to store version or NULL if you don't need
+ *                           it
+ * @param[out] payload_size  Pointer to store payload size or NULL if you don't
+ *                           need it
+ *
+ * @return     true on success, false otherwise
+ */
+bool saved_struct_get_metadata(
+    const char* path,
+    uint8_t* magic,
+    uint8_t* version,
     size_t* payload_size);
 
 #ifdef __cplusplus