Feature: JSON encode / decode support.

[TjazVracko]

Context

Support is required for being able to set settings from a JSON input and also to serialize the settings into a JSON structure.
cJSON will be used as the JSON parsing lib, since it is available in zephyr.

Required steps / Implementation details

• Add changed flag to struct user_setting that is set to true when a setting value is changed (prv_user_settings_set).
• Add iteration functions to existing user_settings.c/.h (See below).
• Add a function to reset a single changed flag and all changed flags (See below).
• Add tests to tests/user_settings to verify this functionality.

Additions to user_settings.h:

/**
 * @brief Start iteration over all user settings
 * 
 * Call this before getting elements with user_settings_list_iter_next.
 * This will reset an in-progress iteration.
 */
void user_settings_iter_start(void);

/**
 * @brief Get next settings ID and KEY in the iteration
 * 
 * @param[out] key The key of the next settings. NULL if no more element exist.
 * @param[out] id The ID of the next settings.NULL if no more element exist.
 */
void user_settings_list_iter_next(char *key, uint16_t* id);

/**
 * @brief Get next settings ID and KEY in the iteration
 * 
 * Only returns settings wil a set changed flag.
 * 
 * @param[out] key The key of the next changed settings. NULL if no more element exist.
 * @param[out] id The ID of the next changed settings.NULL if no more element exist.
 */
void user_settings_list_iter_next_changed(char *key, uint16_t* id);

/* Write docstring here pls. should be obvious */
void user_settings_clear_changed_by_key(char *key);
void user_settings_clear_changed_by_id(uint16_t id);

/**
 * @brief Clears changed on all settings
 */
void user_settings_clear_changed(void);

• Add a new submodule user_settings_json.h/.c
• Implement bellow API.
• Add Kconfig options to enable this module (something like CONFIG_USER_SETTINGS_JSON).
• Add appropriate tests to tests/user_settings_json`

/**
 * @brief Set settings from a flat json structure
 *
 * Does not modify the passed in JSON.
 *
 * Expected structure looks like:
 * {
 *   "s_key_1": <value>,
 *   "s_key_2": <value>,
 *   // ...
 * }
 *
 * @param[in] settings The settings to apply
 *
 * @return int Can this fail? Figure out if any errors can be returned or if this can be void.
 */
int user_settings_set_from_json(const cJSON *settings);

/**
 * @brief Create a JSON with containing only settings marked changed.
 *
 * The caller is expected to free  the created cJSON structure.
 *
 * @param[out] settings Created json
 * @return int This can probably fail with allocation errors. Be thorough please.
 */
int settings user_settings_get_changed_json(cJSON **settings);

/**
 * @brief Create a JSON with containing all settings.
 *
 * The caller is expected to free  the created cJSON structure.
 *
 * @param[out] settings Created json
 * @return int This can probably fail with allocation errors. Be thorough please.
 */
int settings user_settings_get_all_json(cJSON **settings);

Definition of Done

[TjazVracko]

If you see that some API function would make more sense in another form, change it!