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>

applications/services/expansion/expansion_settings.h

@@ -36,7 +36,7 @@ bool expansion_settings_load(ExpansionSettings* settings);
  * @param[in] settings pointer to an ExpansionSettings instance to save settings from.
  * @returns true if the settings were successfully saved, false otherwise.
  */
-bool expansion_settings_save(ExpansionSettings* settings);
+bool expansion_settings_save(const ExpansionSettings* settings);
 
 #ifdef __cplusplus
 }